National Grid (NGG) 20-F 4 Jun 18 2018 FY Annual report (foreign) Financial data

Document And Entity Information

Document And Entity Information	12 Months Ended
	Mar. 31, 2018shares
Document And Entity Information [Abstract]
Entity Registrant Name	NATIONAL GRID PLC
Entity Central Index Key	1,004,315
Current Fiscal Year End Date	--03-31
Document Type	20-F
Document Period End Date	Mar. 31, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	FY
Entity Current Reporting Status	Yes
Entity Well-known Seasoned Issuer	Yes
Entity Common Stock, Shares Outstanding	3,637,747,827
Entity Filer Category	Large Accelerated Filer
Entity Accounting Standard	IFRS
Amendment Flag	false

Consolidated income statement

Consolidated income statement - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018		Mar. 31, 2017		Mar. 31, 2016
Statement [Line Items]
Revenue	£ 15,250		£ 15,035	[1]	£ 13,212	[2]
Operating costs	(11,757)		(11,827)	[1]	(9,987)	[2]
Operating profit	3,493		3,208	[1]	3,225	[2]
Finance income	154		53	[1]	22	[2]
Finance costs	(899)		(1,140)	[1]	(977)	[2]
Share of post-tax results of joint ventures and associates	(40)		63	[1]	59	[2]
Profit before tax	2,708		2,184	[1]	2,329	[2]
Tax	884		(374)	[1]	(427)	[2]
Profit after tax from continuing operations	3,592		1,810	[1]	1,902	[2]
Profit (loss) after tax from discontinued operations	(41)		5,984	[1]	692	[2]
Total profit for the year (continuing and discontinued)	3,551		7,794	[1]	2,594	[2]
Attributable to:
Equity shareholders of the parent	3,550		7,795	[1]	2,591	[2]
Non-controlling interests	£ 1	[3]	£ (1)	[1],[4]	£ 3	[2],[5]
Earnings per share (pence)
Basic earnings per share (continuing) (in GBP per share)	£ 1.038		£ 0.481	[1]	£ 0.504	[2]
Diluted earnings per share (continuing) (in GBP per share)	1.033		0.479	[1]	0.502	[2]
Basic earnings per share (continuing and discontinued) (in GBP per share)	1.026		2.071	[1]	0.687	[2]
Diluted earnings per share (continuing and discontinued) (in GBP per share)	£ 1.021		£ 2.062	[1]	£ 0.684	[2]
Before exceptional items and remeasurements £m
Statement [Line Items]
Revenue	£ 15,250		£ 15,035	[1]	£ 13,212	[2]
Operating costs	(11,793)		(11,262)	[1]	(9,998)	[2]
Operating profit	3,457		3,773	[1]	3,214	[2]
Finance income	154		53	[1]	22	[2]
Finance costs	(1,128)		(1,082)	[1]	(878)	[2]
Share of post-tax results of joint ventures and associates	167		63	[1]	59	[2]
Profit before tax	2,650		2,807	[1]	2,417	[2]
Tax	(589)		(666)	[1]	(604)	[2]
Profit after tax from continuing operations	2,061		2,141	[1]	1,813	[2]
Profit (loss) after tax from discontinued operations	0		606	[1]	576	[2]
Total profit for the year (continuing and discontinued)	2,061		2,747	[1]	2,389	[2]
Attributable to:
Equity shareholders of the parent	2,060		2,747	[1]	2,386	[2]
Non-controlling interests	£ 1	[3]	£ 0	[1],[4]	£ 3	[2],[5]
Earnings per share (pence)
Basic earnings per share (continuing) (in GBP per share)	£ 0.595		£ 0.569		£ 0.480
Diluted earnings per share (continuing) (in GBP per share)	0.593		0.567		0.478
Basic earnings per share (continuing and discontinued) (in GBP per share)	0.595		0.730		0.632
Diluted earnings per share (continuing and discontinued) (in GBP per share)	£ 0.593		£ 0.727		£ 0.629
Exceptional items and remeasurements (see note 4) £m
Statement [Line Items]
Revenue	£ 0		£ 0	[1]	£ 0	[2]
Operating costs	36		(565)	[1]	11	[2]
Operating profit	36		(565)	[1]	11	[2]
Finance income	0		0	[1]	0	[2]
Finance costs	229		(58)	[1]	(99)	[2]
Share of post-tax results of joint ventures and associates	(207)		0	[1]	0	[2]
Profit before tax	58		(623)	[1]	(88)	[2]
Tax	1,473		292	[1]	177	[2]
Profit after tax from continuing operations	1,531		(331)	[1]	89	[2]
Profit (loss) after tax from discontinued operations	(41)		5,378	[1]	116	[2]
Total profit for the year (continuing and discontinued)	1,490		5,047	[1]	205	[2]
Attributable to:
Equity shareholders of the parent	1,490		5,048	[1]	205	[2]
Non-controlling interests	£ 0	[3]	£ (1)	[1],[4]	£ 0	[2],[5]
Earnings per share (pence)
Basic earnings per share (continuing and discontinued) (in GBP per share)	£ 0.431		£ 1.341		£ 0.055
Diluted earnings per share (continuing and discontinued) (in GBP per share)	£ 0.428		£ 1.335		£ 0.055
[1]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).
[2]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).
[3]	The non-controlling interests for the year ended 31 March 2018 relate to continuing operations.
[4]	The non-controlling interests for the year ended 31 March 2017 relate to discontinued operations.
[5]	The non-controlling interests for the year ended 31 March 2016 relate to both continuing and discontinued operations.

Consolidated income statement (

Consolidated income statement (Parenthetical) £ in Millions	Mar. 31, 2017GBP (£)$ / £	Mar. 31, 2018GBP (£)$ / ££ / shares	Mar. 31, 2017GBP (£)$ / ££ / shares		Mar. 31, 2016GBP (£)£ / shares		Mar. 31, 2015GBP (£)£ / shares	Mar. 31, 2014GBP (£)£ / shares
Disclosure of operating segments [line items]
Increase (decrease) in revenues		£ 215
Revenue		15,250	£ 15,035	[1]	£ 13,212	[2]
Operating costs		11,757	11,827	[1]	9,987	[2]
Increase (decrease) in operating costs		(70)
Storm costs		142
Increase (decrease) in net finance costs		(55)
Finance costs before exceptional items			974
Finance costs		(745)	(1,087)		(955)
Derivative assets	£ 1,813	1,724	1,813
Increase (decrease) in share of post-tax results of joint ventures and associates		104
Share of post-tax results of joint ventures and associates		(40)	63	[1]	59	[2]
Tax expense (income), continuing operations		(884)	374	[1]	427	[2]
Decrease in tax charge on profits before exceptional items and remeasurements		77
Finance costs		899	1,140	[1]	977	[2]
Adjusted profit (loss) from continuing operations		£ 2,060	£ 2,141		£ 1,812		£ 1,675	£ 1,465
Adjusted earnings (loss) per share (gbp per share) | £ / shares		£ 0.595	£ 0.569		£ 0.480		£ 0.439	£ 0.384
Increase (decrease) in adjusted earnings per share (gbp per share) | £ / shares		£ 0.026
Increase (decrease) in adjusted earnings per share, in percent		5.00%
Exchange rates, weighted average (income statement) (usd per gbp) | $ / £		1.36	1.28
Exchange rates, weighted average (income statement) percent change		6.00%
Exchange rates, year-end (statement of financial position) (gbp per share) | $ / £	1.25	1.40	1.25
Exchange rates, year-end (statement of financial position), percent		12.00%
US Regulated
Disclosure of operating segments [line items]
Increase (decrease) in revenues		£ 341
Revenue		9,272	£ 8,931		£ 7,493
UK Electricity Transmission
Disclosure of operating segments [line items]
Increase (decrease) in revenues		(285)
Revenue		4,126	4,410		3,957
NG Ventures and Other
Disclosure of operating segments [line items]
Revenue		770	713		824
Share of post-tax results of joint ventures and associates		49	63		59
Exceptional items and remeasurements
Disclosure of operating segments [line items]
Revenue		0	0	[1]	0	[2]
Operating costs		(36)	565	[1]	(11)	[2]
Increase (decrease) in operating costs		(601)
Share of post-tax results of joint ventures and associates		(207)	0	[1]	0	[2]
Tax expense (income), continuing operations		(1,473)	(292)	[1]	(177)	[2]
Final settlement of LIPA MSA Transition		26
Finance costs		(229)	58	[1]	99	[2]
Gain on put option		180	(139)		(89)
Before exceptional items and remeasurements
Disclosure of operating segments [line items]
Revenue		15,250	15,035	[1]	13,212	[2]
Operating costs		11,793	11,262	[1]	9,998	[2]
Increase (decrease) in operating costs		531
Share of post-tax results of joint ventures and associates		167	63	[1]	59	[2]
Tax expense (income), continuing operations		589	666	[1]	604	[2]
Finance costs		1,128	1,082	[1]	878	[2]
Before exceptional items and remeasurements | NG Ventures and Other
Disclosure of operating segments [line items]
Share of post-tax results of joint ventures and associates		44	63		59
Revenue
Disclosure of operating segments [line items]
Net foreign exchange gain		536
Adjusted operating profit
Disclosure of operating segments [line items]
Net foreign exchange gain		98
Operating profit
Disclosure of operating segments [line items]
Net foreign exchange gain		73
Unallocated amounts | NG Ventures and Other
Disclosure of operating segments [line items]
Increase (decrease) in revenues		63
Revenue		776	£ 713		£ 824
Quadgas HoldCo Limited
Disclosure of operating segments [line items]
Revenue		1,468
Share of post-tax results of joint ventures and associates		£ 124
Proportion of ownership interest in associate	39.00%	39.00%
Tax expense (income), continuing operations		£ 76
Proportion of ownership interest in associate, put/call option equity interest		14.00%	14.00%
Proportion of ownership interest in associate, excluding put/call option equity interest		25.00%
Derivative Financial Instruments | Exceptional items and remeasurements
Disclosure of operating segments [line items]
Finance costs		£ (119)	£ 58
Commodity contract derivatives
Disclosure of operating segments [line items]
Derivative assets	£ 106	69	£ 106
Commodity contract derivatives | Exceptional items and remeasurements
Disclosure of operating segments [line items]
Remeasurements – commodity contract derivatives		10
Further Acquisition Agreement derivative
Disclosure of operating segments [line items]
Derivative assets		110
Further Acquisition Agreement derivative | Exceptional items and remeasurements
Disclosure of operating segments [line items]
Gain on put option		£ 110
[1]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).
[2]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).

Consolidated statement of compr

Consolidated statement of comprehensive income - GBP (£) £ in Millions		12 Months Ended
		Mar. 31, 2018	Mar. 31, 2017		Mar. 31, 2016
Disclosure of analysis of other comprehensive income by item [line items]
Profit after tax from continuing operations		£ 3,592	£ 1,810	[1]	£ 1,902	[2]
Items from continuing operations that will never be reclassified to profit or loss:
Remeasurement gains of pension assets and post-retirement benefit obligations		1,313	423		410
Share of other comprehensive income of associates, net of tax	[3]	142	0		0
Tax on items that will never be reclassified to profit or loss		(530)	(277)		(95)
Total items from continuing operations that will never be reclassified to profit or loss		925	146		315
Items from continuing operations that may be reclassified subsequently to profit or loss:
Exchange adjustments		(505)	346		69
Net gains in respect of cash flow hedges		19	70		88
Transferred to profit or loss in respect of cash flow hedges		(3)	(6)		26
Net (losses)/gains on available-for-sale investments		(30)	81		43
Transferred to profit or loss on sale of available-for-sale investments		(73)	(25)		0
Share of net gains taken to equity of associates		5	0		0
Tax on items that may be reclassified subsequently to profit or loss	[3]	33	(34)		(39)
Total items from discontinued operations that may be reclassified subsequently to profit or loss		(554)	432		187
Other comprehensive income for the year, net of tax from continuing operations		371	578		502
Other comprehensive income for the year, net of tax from discontinued operations		0	42		71
Other comprehensive income for the year, net of tax		371	620		573
Total comprehensive income for the year from continuing operations		3,963	2,388		2,404
Total comprehensive income for the year from discontinued operations		(41)	6,026		763
Total comprehensive income for the year		3,922	8,414		3,167
Comprehensive income, attributable to owners of parent		3,922	8,415		3,164
Comprehensive income, attributable to non-controlling interests		0	(1)		3
Other comprehensive income, net of tax, gains (losses) on remeasurements of defined benefit plans		925	146		315
Continuing operations
Items from continuing operations that may be reclassified subsequently to profit or loss:
Comprehensive income, attributable to owners of parent		3,963	2,389		2,403
Comprehensive income, attributable to non-controlling interests		0	(1)		1
Discontinued operations
Items from continuing operations that will never be reclassified to profit or loss:
Remeasurement gains of pension assets and post-retirement benefit obligations			(75)		129
Tax on items that will never be reclassified to profit or loss			13		(30)
Total items from continuing operations that will never be reclassified to profit or loss			(62)		99
Items from continuing operations that may be reclassified subsequently to profit or loss:
Net gains in respect of cash flow hedges			(106)		(38)
Transferred to profit or loss in respect of cash flow hedges			233		3
Tax on items that may be reclassified subsequently to profit or loss			(23)		7
Total items from discontinued operations that may be reclassified subsequently to profit or loss			104		(28)
Other comprehensive income for the year, net of tax			42		71
Total comprehensive income for the year			6,026		763
Comprehensive income, attributable to owners of parent		(41)	6,026		761
Comprehensive income, attributable to non-controlling interests		0	0		2
UK Gas Distribution (Cadent)
Items from continuing operations that may be reclassified subsequently to profit or loss:
Other comprehensive income, net of tax, gains (losses) on remeasurements of defined benefit plans		142	0		0
Gains (losses) on cash flow hedges, net of tax		£ 5	£ 0		£ 0
[1]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).
[2]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).
[3]	The share of other comprehensive income of associates relates to items of other comprehensive income of Cadent (investment through Quadgas HoldCo Limited), comprising £142 million (2017: £nil; 2016: £nil) remeasurement gains on pension assets and post-retirement benefit obligations and a £5 million (2017: £nil; 2016: £nil) net gain in respect of cash flow hedges. Both items are shown net of tax.

Consolidated statement of comp5

Consolidated statement of comprehensive income (Parenthetical) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Statement of comprehensive income [abstract]
Other comprehensive income, net of tax, gains (losses) on remeasurements of defined benefit plans	£ 925	£ 146	£ 315
Exchange adjustments	(505)	346	69
Remeasurement gains of pension assets and post-retirement benefit obligations	1,313	348	539
Net gains in respect of cash flow hedges	£ 19	£ 70	£ 88

Consolidated statement of chang

Consolidated statement of changes in equity - GBP (£) £ in Millions		Total		Total shareholders’ equity £m	Share capital £m	Share premium account £m	Retained earnings £m	Other equity reserves £m	[1]	Non- controlling interests £m
Equity, beginning balance at Mar. 31, 2015		£ 11,974		£ 11,962	£ 443	£ 1,331	£ 14,870	£ (4,682)		£ 12
Profit/(loss) for the year		2,594	[2]	2,591			2,591	0		3
Other comprehensive income/(loss) for the year		573		573			414	159		0
Total comprehensive income for the year		3,167		3,164			3,005	159		3
Equity dividends		(1,337)		(1,337)			(1,337)
Scrip dividend	[3]	(1)		(1)	4	(5)
Purchase of treasury shares		(267)		(267)			(267)
Issue of treasury shares		16		16			16
Purchase of own shares		(6)		(6)			(6)
Other movements in non-controlling interests		(5)								(5)
Share-based payments		22		22			22
Tax on share-based payments		2		2			2
Equity, ending balance at Mar. 31, 2016		13,565		13,555	447	1,326	16,305	(4,523)		10
Profit/(loss) for the year		7,794	[4]	7,795			7,795	0		(1)
Other comprehensive income/(loss) for the year		620		620			84	536		0
Total comprehensive income for the year		8,414		8,415			7,879	536		(1)
Equity dividends		(1,463)		(1,463)			(1,463)
Scrip dividend	[3]	0		0	2	(2)
Purchase of treasury shares		(189)		(189)			(189)
Issue of treasury shares		18		18			18
Purchase of own shares		(6)		(6)			(6)
Other movements in non-controlling interests		7								7
Share-based payments		35		35			35
Tax on share-based payments		3		3			3
Equity, ending balance at Mar. 31, 2017		20,384	[5]	20,368	449	1,324	22,582	(3,987)		16
Profit/(loss) for the year		3,551		3,550			3,550	0		1
Other comprehensive income/(loss) for the year		371		372			925	(553)		(1)
Total comprehensive income for the year		3,922		3,922			4,475	(553)		0
Equity dividends		(4,487)		(4,487)			(4,487)
Scrip dividend	[3]	0		0	3	(3)
Purchase of treasury shares		(1,017)		(1,017)			(1,017)
Issue of treasury shares		33		33			33
Purchase of own shares		(5)		(5)			(5)
Share-based payments		16		16			16
Tax on share-based payments		2		2			2
Equity, ending balance at Mar. 31, 2018	[6]	£ 18,848		£ 18,832	£ 452	£ 1,321	£ 21,599	£ (4,540)		£ 16
[1]	For further details of other equity reserves, see note 26.
[2]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).
[3]	Included within the share premium account are costs associated with scrip dividends.
[4]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).
[5]	Comparative amounts have been represented to reflect the reclassification of commodity derivative contracts from trade and other receivables and payables, and from other non-current assets and liabilities to derivative financial assets and derivative financial liabilities (see note 16).
[6]	Refer to note 7 for the effect of the share consolidation and the special dividend.

Consolidated statement of chan7

Consolidated statement of changes in equity (Parenthetical) £ in Millions	12 Months Ended
	Mar. 31, 2018GBP (£)£ / shares
Summary Of Dividends [Line Items]
Proposed dividend (gbp per share)	£ 0.3044
Dividend declared during the year (gbp per share)	£ 0.4593
Increase in dividends	3.70%
Cash dividends paid (usd/gbp per share)	£ 128.965
Dividends paid | £	£ 4,487
Special dividend
Summary Of Dividends [Line Items]
Cash dividends paid (usd/gbp per share)	£ 84.375
Dividends paid | £	£ 3,171

Consolidated statement of finan

Consolidated statement of financial position - GBP (£) £ in Millions	Mar. 31, 2018		Mar. 31, 2017	[1]	Mar. 31, 2016	Mar. 31, 2015
Non-current assets
Goodwill	£ 5,444		£ 6,096
Other intangible assets	899		923
Property, plant and equipment	39,853		39,825
Other non-current assets	115		69
Pension assets	1,409		603
Financial and other investments	899		1,100
Investments in joint ventures and associates	2,168		2,083		£ 397
Derivative financial assets	1,319		1,567
Total non-current assets	52,106		52,266
Current assets
Inventories and current intangible assets	341		403
Trade and other receivables	2,798		2,728
Current tax assets	114		317
Financial and other investments	2,694		8,741		2,998
Derivative financial assets	405		246
Cash and cash equivalents	329		1,139		127
Total current assets	6,681		13,574
Total assets	58,787		65,840
Current liabilities
Borrowings	(4,447)		(5,496)
Derivative financial liabilities	(401)		(1,147)
Trade and other payables	(3,453)		(3,345)
Current tax liabilities	(123)		(107)
Provisions	(273)		(416)
Total current liabilities	(8,697)		(10,511)
Non-current liabilities
Borrowings	(22,178)		(23,142)
Derivative financial liabilities	(660)		(1,246)
Other non-current liabilities	(1,317)		(1,370)
Deferred tax liabilities	(3,636)		(4,479)
Pensions and other post-retirement benefit obligations	(1,672)		(2,536)
Provisions	(1,779)		(2,172)
Total non-current liabilities	(31,242)		(34,945)
Total liabilities	(39,939)		(45,456)
Net assets	18,848		20,384
Equity
Share capital	452		449
Share premium account	1,321		1,324
Retained earnings	21,599		22,582
Other equity reserves	(4,540)		(3,987)		(4,523)	£ (4,682)
Total shareholders’ equity	18,832		20,368
Non-controlling interests	16		16
Total equity	£ 18,848	[2]	£ 20,384		£ 13,565	£ 11,974
[1]	Comparative amounts have been represented to reflect the reclassification of commodity derivative contracts from trade and other receivables and payables, and from other non-current assets and liabilities to derivative financial assets and derivative financial liabilities (see note 16).
[2]	Refer to note 7 for the effect of the share consolidation and the special dividend.

Consolidated statement of fina9

Consolidated statement of financial position Investments and other non-current assets (Parenthetical) £ in Millions	12 Months Ended
	Mar. 31, 2018GBP (£)
Disclosure of operating segments [line items]
Decrease in investments and other non-current assets	£ 70
Investments and other non-current assets	3,182
Disposal of investments	134
Investments in joint ventures and associates	129
Recognition of non-current corporation tax asset	51
Quadgas HoldCo Limited
Disclosure of operating segments [line items]
Net reduction in investments accounted for using equity method	(17)
Shares In Dominion
Disclosure of operating segments [line items]
Disposal of investments	£ 173

Consolidated statement of fin10

Consolidated statement of financial position Inventories and current intangible assets, and trade and other receivables (Parenthetical) £ in Millions	12 Months Ended
	Mar. 31, 2018GBP (£)
Statement of financial position [abstract]
Increase (decrease) in inventories and current intangible assets and trading and other receivables, excluding foreign exchange	£ 286
Increase (decrease) in inventories and current intangible assets and trading and other receivables, foreign exchange	278
Current inventories, current intangible assets and trade and other current receivables	3,139
Increase (decrease) in current intangible assets	£ (30)

Consolidated statement of fin11

Consolidated statement of financial position Current tax balances and deferred tax balances (Parenthetical) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Statement of financial position [abstract]
Current tax liabilities	£ 9
Increase (decrease) in net current tax asset (liability)	(219)
Current tax assets		£ 210
Increase (decrease) in deferred tax liability (asset)	(843)
Deferred tax liabilities	£ 3,636	£ 4,479	[1]
[1]	Comparative amounts have been represented to reflect the reclassification of commodity derivative contracts from trade and other receivables and payables, and from other non-current assets and liabilities to derivative financial assets and derivative financial liabilities (see note 16).

Consolidated statement of fin12

Consolidated statement of financial position Trade and other payables (Parenthetical) £ in Millions	12 Months Ended
	Mar. 31, 2018GBP (£)
Statement of financial position [abstract]
Increase (decrease) trade and other payables, excluding foreign exchange	£ 313
Increase (decrease) trade and other payables, foreign exchange	209
Trade and other payables	£ 3,453

Consolidated statement of fin13

Consolidated statement of financial position Provisions and other non current liabilities (Parenthetical) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disclosure of operating segments [line items]
Increase (decrease) other provisions and other non-current liabilities excluding foreign exchange impact	£ (324)
Increase (decrease) other provisions and other non-current liabilities, foreign exchange impact	265
Other provisions and other non-current liabilities	3,369
Increase (decrease) in other provisions	(341)
Professional fees expense	93
Utilised	268	£ 227
Other provisions	2,052	2,588
Other environment related provision
Disclosure of operating segments [line items]
Utilised	75	110
Other provisions	1,531	1,721
Voluntary Distribution
Disclosure of operating segments [line items]
Utilised		33
Other provisions	£ 150	£ 117

Consolidated statement of fin14

Consolidated statement of financial position Net pension and other post-retirement obligations (Parenthetical) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disclosure of defined benefit plans [line items]
Net defined benefit liability (asset), beginning balance	£ (1,933)	£ (2,585)
Exchange adjustments	175	(345)
Current service cost	(193)	(232)
Net interest cost	(65)	(105)
Curtailments and other	(35)
Remeasurement gains of pension assets and post-retirement benefit obligations	1,313	348
Employer contributions	475	1,073
Net defined benefit liability (asset), ending balance	(263)	(1,933)
Fair value of plan assets	23,858
Plan liabilities	(24,121)
Funding surplus/(deficit)	(263)	(1,933)
Plan assets
Disclosure of defined benefit plans [line items]
Net defined benefit liability (asset), beginning balance	24,375	26,434
Exchange adjustments	(970)	1,049
Net interest cost	(710)	(952)
Remeasurement gains of pension assets and post-retirement benefit obligations	568
Employer contributions	475	1,073
Net defined benefit liability (asset), ending balance	23,858	24,375
Present value of defined benefit obligation
Disclosure of defined benefit plans [line items]
Net defined benefit liability (asset), beginning balance	(26,230)	(28,952)
Exchange adjustments	1,145	(1,394)
Current service cost	193	232
Net interest cost	775	1,057
Remeasurement gains of pension assets and post-retirement benefit obligations	745
Net defined benefit liability (asset), ending balance	(24,054)	(26,230)
Actuarial gains/(losses) – financial assumptions	174	(3,377)
Actuarial gains – demographic assumptions	671	225
UK Pensions
Disclosure of defined benefit plans [line items]
Net defined benefit liability (asset), beginning balance	(156)	(15)
Exchange adjustments	0	0
Current service cost	(49)	(76)
Net interest cost	(3)	0
Curtailments and other	(15)
Remeasurement gains of pension assets and post-retirement benefit obligations	1,177	(541)
Employer contributions	150	528
Net defined benefit liability (asset), ending balance	1,104	(156)
Fair value of plan assets	15,330	15,489
Plan liabilities	(14,226)
Funding surplus/(deficit)	1,104	(156)
UK Pensions | Plan assets
Disclosure of defined benefit plans [line items]
Net defined benefit liability (asset), beginning balance	15,489	19,401
Exchange adjustments	0	0
Net interest cost	(363)	(615)
Remeasurement gains of pension assets and post-retirement benefit obligations	103
Employer contributions	150	528
Net defined benefit liability (asset), ending balance	15,330	15,489
UK Pensions | Present value of defined benefit obligation
Disclosure of defined benefit plans [line items]
Net defined benefit liability (asset), beginning balance	(15,645)	(19,416)
Exchange adjustments	0	0
Current service cost	49	76
Net interest cost	366	615
Remeasurement gains of pension assets and post-retirement benefit obligations	1,074
Net defined benefit liability (asset), ending balance	(14,226)	(15,645)
Actuarial gains/(losses) – financial assumptions	604	(3,751)
Actuarial gains – demographic assumptions	565	214
US Pension and Post-employment
Disclosure of defined benefit plans [line items]
Net defined benefit liability (asset), beginning balance	(1,777)
Exchange adjustments	175
Current service cost	(144)
Net interest cost	(62)
Curtailments and other	(20)
Employer contributions	325
Net defined benefit liability (asset), ending balance	(1,367)	£ (1,777)
Fair value of plan assets	8,528
Plan liabilities	(9,895)
Funding surplus/(deficit)	(1,367)
US Pension and Post-employment | Plan assets
Disclosure of defined benefit plans [line items]
Remeasurement gains of pension assets and post-retirement benefit obligations	465
US Pension and Post-employment | Present value of defined benefit obligation
Disclosure of defined benefit plans [line items]
Remeasurement gains of pension assets and post-retirement benefit obligations	(329)
Actuarial gains/(losses) – financial assumptions	£ (430)

Goodwill and other intangible a

Goodwill and other intangible assets (Parenthetical) £ in Millions	12 Months Ended
	Mar. 31, 2018GBP (£)
Disclosure of detailed information about intangible assets [line items]
Increase (decrease) in intangible assets and goodwill	£ (676)
Intangible assets and goodwill	6,343
Exchange adjustments	707
Software
Disclosure of detailed information about intangible assets [line items]
Additions	173
Amortisation, intangible assets other than goodwill	£ 138

Consolidated statement of fin16

Consolidated statement of financial position Property, plant and equipment (Parenthetical) £ in Millions	12 Months Ended
	Mar. 31, 2018GBP (£)
Statement of financial position [abstract]
Increase (decrease) in property, plant and equipment	£ 28
Property, plant and equipment	39,853
Additions	3,901
Depreciation charge for the year	1,392
Disposals	45
Exchange adjustments	£ 2,428

Consolidated cash flow statemen

Consolidated cash flow statement - GBP (£)		12 Months Ended
		Mar. 31, 2018		Mar. 31, 2017		Mar. 31, 2016
Cash flows from operating activities
Total operating profit from continuing operations		£ 3,493,000,000		£ 3,208,000,000	[1]	£ 3,225,000,000	[2]
Adjustments for:
Exceptional items and remeasurements		(36,000,000)		565,000,000		(11,000,000)
Depreciation, amortisation and impairment		1,530,000,000		1,481,000,000		1,311,000,000
Share-based payments charge		16,000,000		32,000,000		21,000,000
Gain on exchange of associate for available-for-sale investment		0		0		(49,000,000)
Changes in working capital		118,000,000		151,000,000		416,000,000
Changes in provisions		(206,000,000)		(181,000,000)		(58,000,000)
Changes in pensions and other post-retirement benefit obligations		(239,000,000)		(768,000,000)		(293,000,000)
Cash flows relating to exceptional items		26,000,000		(36,000,000)		(40,000,000)
Cash generated from operations – continuing operations		4,702,000,000		4,452,000,000		4,522,000,000
Tax recovered/(paid)		8,000,000		(132,000,000)		(230,000,000)
Net cash inflow from operating activities – continuing operations		4,710,000,000		4,320,000,000		4,292,000,000
Net cash (used in)/inflow from operating activities – discontinued operations		(207,000,000)		909,000,000		1,076,000,000
Cash flows from investing activities
Acquisition of investments		(2,000,000)		0		0
Investments in joint ventures and associates		(129,000,000)		(76,000,000)		(116,000,000)
Loans to joint ventures and associates		(68,000,000)		(61,000,000)		0
Disposal of investments		134,000,000		0		0
Disposal of UK Gas Distribution		(20,000,000)		5,454,000,000		0
Purchases of intangible assets		(173,000,000)		(223,000,000)		(196,000,000)
Purchases of property, plant and equipment		(3,738,000,000)		(3,296,000,000)		(2,855,000,000)
Disposals of property, plant and equipment		10,000,000		18,000,000		4,000,000
Dividends received from joint ventures and associates		213,000,000		99,000,000		72,000,000
Interest received		57,000,000		51,000,000		23,000,000
Net movements in short-term financial investments	[3]	5,953,000,000		(5,600,000,000)		(391,000,000)
Net cash flow from/(used in) investing activities – continuing operations		2,237,000,000		(3,634,000,000)		(3,459,000,000)
Net cash flow used in investing activities – discontinued operations		0		(680,000,000)		(577,000,000)
Cash flows from financing activities
Purchase of treasury shares		(1,017,000,000)		(189,000,000)		(267,000,000)
Proceeds from issue of treasury shares		33,000,000		18,000,000		16,000,000
Purchase of own shares		(5,000,000)		(6,000,000)		(6,000,000)
Proceeds received from loans		1,941,000,000		2,463,000,000		2,726,000,000
Repayment of loans		(2,156,000,000)		(1,616,000,000)		(896,000,000)
Net movements in short-term borrowings and derivatives		772,000,000		(90,000,000)		730,000,000
Interest paid		(853,000,000)		(839,000,000)		(711,000,000)
Dividends paid to shareholders		(4,487,000,000)		(1,463,000,000)		(1,337,000,000)
Net cash flow used in financing activities – continuing operations		(7,316,000,000)		(1,542,000,000)		(1,205,000,000)
Net cash flow (used in)/from financing activities – discontinued operations		(231,000,000)		1,611,000,000		(123,000,000)
Net (decrease)/increase in cash and cash equivalents		(807,000,000)		984,000,000		4,000,000
Disposal of bank overdraft in UK Gas Distribution		0		15,000,000		0
Exchange movements		(3,000,000)		16,000,000		4,000,000
Cash and cash equivalents, beginning balance		1,139,000,000	[3]	124,000,000	[3]	116,000,000
Cash and cash equivalents, ending balance	[3]	329,000,000		1,139,000,000		124,000,000
Bank overdrafts		£ 0		£ 0		£ 3,000,000
[1]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).
[2]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).
[3]	Net of bank overdrafts of £nil (2017: £nil; 2016: £3 million).

Consolidated cash flow statem18

Consolidated cash flow statement (reconciliation of net debt to cash flows) (parenthetical) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Continuing and Discontinued Operations [Line Items]
Cash generated	£ 4,702	£ 4,452
Business net cash flow	397	1,196
Net interest paid	(796)	(1,167)
Tax recovered	8	(132)
Ordinary dividends paid and scrip buybacks	(1,494)	(1,652)
Return of capital	(4,010)	0
Disposal of UK Gas Distribution	(20)	11,344
Other cash movements	239	110
Non-cash movements	1,948	(2,782)
(Increase)/decrease in net debt	(3,728)	6,051
Net debt (net of related derivative financial instruments) at start of year	(19,274)	(25,325)
Net debt (net of related derivative financial instruments) at end of year	(23,002)	(19,274)
Continuing and discontinued operations
Continuing and Discontinued Operations [Line Items]
Cash generated	4,495	5,439
Net capital investment	(4,098)	(4,243)
Net interest paid	(796)	(1,955)
Tax recovered	8	(210)
Continuing operations
Continuing and Discontinued Operations [Line Items]
Net capital investment	(4,098)	(3,638)
Net interest paid	(796)	(788)
Tax recovered	8	(132)
Discontinued operations
Continuing and Discontinued Operations [Line Items]
Cash generated	(207)	987
Net capital investment	0	(605)
Net interest paid	0	(1,167)
Tax recovered	£ 0	£ (78)

Consolidated cash flow statem19

Consolidated cash flow statement (Parenthetical) £ in Millions	Jun. 02, 2018GBP (£)	Mar. 31, 2018GBP (£)	Mar. 31, 2017GBP (£)
Continuing and Discontinued Operations [Line Items]
Cash flows from operations		£ (4,702)	£ (4,452)
Increase (decrease) in cash flows		250
Outflow from changes in pensions and other post-retirement obligations		239	768
Change in outflow from changes in pensions and other post-retirement obligations		529
Loans to joint ventures and associates		197
Purchases of intangibles and property, plant and equipment net of disposals		3,901
Net foreign exchange loss		133
Interest paid		796	1,167
Tax recovered		8	(132)
Dividends paid		4,487	1,463
Decrease in dividends		147
Share buybacks related to repurchase of scrip uptake		178
Decrease in share buybacks related to repurchase of the scrip uptake		11
Return to shareholder's of gain on disposal		4,010	0
Stock consolidation ratio	0.9167
Share buybacks		839
Cash proceeds received from disposal		(20)	5,454
Net debt deconsolidated on disposal of subsidiary		0	5,890
Disposal of investments		134	0
Dividends received from joint ventures and associates		213	99
Increase in dividends from joint ventures and associates		114
Net debt		(23,002)	(19,274)
Continuing operations
Continuing and Discontinued Operations [Line Items]
Net capital investment		4,098	3,638
Increase (decrease) in net capital investment		460
Interest paid		796	788
Increase in net interest paid		8
Tax recovered		8	(132)
Discontinued operations
Continuing and Discontinued Operations [Line Items]
Cash flows from operations		207	(987)
Increase (decrease) in cash flows		(1,194)
Net capital investment		0	605
Interest paid		0	1,167
Debt buyback costs			1,052
Tax recovered		0	£ (78)
Dividends
Continuing and Discontinued Operations [Line Items]
Dividends paid		1,316
Special dividend
Continuing and Discontinued Operations [Line Items]
Dividends paid	£ 3,171
US
Continuing and Discontinued Operations [Line Items]
Income tax refund		46
UK
Continuing and Discontinued Operations [Line Items]
Income tax refund		67
Disposal of investments		£ 126

Basis of preparation and recent

Basis of preparation and recent accounting developments	12 Months Ended
	Mar. 31, 2018
Accounting Policies, Changes In Accounting Estimates And Errors [Abstract]
Basis of preparation and recent accounting pronouncements	Basis of preparation and recent accounting developments Accounting policies describe our approach to recognising and measuring transactions and balances in the year. Accounting policies applicable across the financial statements are shown below. Accounting policies that are specific to a component of the financial statements have been incorporated into the relevant note. This section also shows areas of judgement and key sources of estimation uncertainty in these financial statements. In addition, we summarise new IASB and EU endorsed accounting standards, amendments and interpretations and whether these are effective in 2019 or later years, explaining how significant changes are expected to affect our reported results. National Grid’s principal activities involve the transmission and distribution of electricity and gas in Great Britain and northeastern US. The Company is a public limited liability company incorporated and domiciled in England and Wales, with its registered office at 1–3 Strand, London WC2N 5EH. The Company, National Grid plc, which is the ultimate parent of the Group, has its primary listing on the London Stock Exchange and is also quoted on the New York Stock Exchange. These consolidated financial statements were approved for issue by the Board on 16 May 2018. These consolidated financial statements have been prepared in accordance with International Accounting Standards (IAS) and International Financial Reporting Standards (IFRS) and related interpretations as issued by the IASB and IFRS as adopted by the EU. They are prepared on the basis of all IFRS accounting standards and interpretations that are mandatory for periods ended 31 March 2018 and in accordance with the Companies Act 2006 applicable to companies reporting under IFRS and Article 4 of the EU IAS Regulation. The comparative financial information has also been prepared on this basis. The consolidated financial statements have been prepared on a historical cost basis, except for the recording of pension assets and liabilities, the revaluation of derivative financial instruments and certain commodity contracts and investments classified as available-for-sale. These consolidated financial statements are presented in pounds sterling, which is also the functional currency of the Company. The notes to the financial statements have been prepared on a continuing basis unless otherwise stated. Our income statement and segmental analysis separately identify financial results before and after exceptional items and remeasurements. The Directors believe that presentation of the results in this way is relevant to an understanding of the Group’s financial performance. Presenting financial results before exceptional items and remeasurements is consistent with the way that financial performance is measured by management and reported to the Board and Executive Committee and aids the comparability of reported financial performance from year to year in this context. Further, this year we have adopted a columnar presentation as we consider it improves the clarity of the presentation, and is consistent with the way that financial performance is measured by management and reported to the Board and Executive Committee, and better enables users of the financial statements to understand the results. The inclusion of total profit for the period from continuing operations before exceptional items and remeasurements forms part of the incentive target set annually for remunerating certain Executive Directors. Accordingly we believe it is important for users of the financial statements to understand how this compares to our results on a statutory basis and year-on-year. The preparation of financial statements requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosures of contingent assets and liabilities and the reported amounts of revenue and expenses during the reporting period (see accounting policy E). A. Going concern The Directors considered it appropriate to prepare the financial statements on a going concern basis, having considered the Company’s cash flow forecasts with respect to business planning and treasury management activities. The going concern basis presumes that the Group has adequate resources to remain in operation, and that the Directors intend it to do so, for at least one year from the date the financial statements are signed. B. Basis of consolidation The consolidated financial statements incorporate the results, assets and liabilities of the Company and its subsidiaries, together with a share of the results, assets and liabilities of joint operations. The Group accounts for joint ventures and associates using the equity method of accounting, where the investment is carried at cost plus post-acquisition changes in the share of net assets of the joint venture or associate, less any provision for impairment. A subsidiary is defined as an entity controlled by the Group. Control is achieved where the Group is exposed to, or has the rights to, variable returns from its involvement with the entity and has the power to affect those returns through its power over the entity. Losses in excess of the consolidated interest in joint ventures and associates are not recognised, except where the Company or its subsidiaries have made a commitment to make good those losses. Where necessary, adjustments are made to bring the accounting policies used in the individual financial statements of the Company, subsidiaries, joint operations, joint ventures and associates into line with those used by the Group in its consolidated financial statements under IFRS. Intercompany transactions are eliminated. The results of subsidiaries (other than relating to UK Gas Distribution as described in C below), joint operations, joint ventures and associates acquired or disposed of during the year are included in the consolidated income statement from the effective date of acquisition or up to the effective date of disposal, as appropriate. Acquisitions are accounted for using the acquisition method, where the purchase price is allocated to the identifiable assets acquired and liabilities assumed on a fair value basis and the remainder recognised as goodwill. C. Disposal of UK Gas Distribution As described further in note 9, on 8 December 2016 the Group entered into a sale and purchase agreement to dispose of a 61% controlling stake in the UK Gas Distribution business. The disposal completed on 31 March 2017 and the Group has retained a 39% interest in the business. As a result, all assets and liabilities of UK Gas Distribution were deemed to be disposed of and a 39% interest reacquired. The 39% retained interest is classified as an associate on the basis that the Group retains significant influence over the business through its retained stake. The Group has the ability to appoint 4 of the 12 directors on the board of Quadgas HoldCo Limited. In addition, the Group entered into a Further Acquisition Agreement (FAA) over a further 14% interest. Refer to note 15 for further details. The Group classified UK Gas Distribution as held for sale as of 8 December 2016, when it became highly probable that the value of the business to the Group would be recovered through sale rather than continuing ownership. As UK Gas Distribution represents a separate major line of business, the business was classified as a discontinued operation in the 2016/17 consolidated income statement. This continues to be reflected in the consolidated income statement and the consolidated statement of comprehensive income, as well as earnings per share (EPS) split between continuing and discontinued operations. In the current year, any true-ups relating to the disposal of the controlling stake are recorded within discontinued operations. 1. Basis of preparation and recent accounting developments continued D. Foreign currencies Transactions in currencies other than the functional currency of the Company or subsidiary concerned are recorded at the rates of exchange prevailing on the dates of the transactions. At each reporting date, monetary assets and liabilities that are denominated in foreign currencies are retranslated at closing exchange rates. Non-monetary assets are not retranslated unless they are carried at fair value. Gains and losses arising on the retranslation of monetary assets and liabilities are included in the income statement, except where the application of hedge accounting requires inclusion in other comprehensive income see note 16. On consolidation, the assets and liabilities of operations that have a functional currency different from the Company’s functional currency of pounds sterling, principally our US operations that have a functional currency of US dollars, are translated at exchange rates prevailing at the reporting date. Income and expense items are translated at the average exchange rates for the period where these do not differ materially from rates at the date of the transaction. Exchange differences arising are recognised in other comprehensive income and transferred to the consolidated translation reserve within other equity reserves see note 26. E. Areas of judgement and key sources of estimation uncertainty The preparation of financial statements requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosures of contingent assets and liabilities, and the reported amounts of revenue and expenses during the reporting period. Actual results could differ from these estimates. Information about such judgements and estimations is contained in the notes to the financial statements, and the key areas are summarised below. Areas of judgement that have the most significant effect on the amounts recognised in the financial statements are as follows: • categorisation of certain items as exceptional items and the definition of adjusted earnings see notes 4 and 7. Key sources of estimation uncertainty that have a significant risk of causing a material adjustment to the carrying amounts of assets and liabilities within the next financial year are as follows: • valuation of liabilities for pensions and other post-retirement benefits see note 23; and • the discount rate and cash flows applied in determining the environmental provisions see note 24. In order to illustrate the impact that changes in assumptions could have on our results and financial position, we have included sensitivity analyses in note 33. F. Accounting policy choices IFRS provides certain options available within accounting standards. Choices we have made, and continue to make, include the following: • Presentational formats: we use the nature of expense method for our income statement and aggregate our statement of financial position to net assets and total equity. In the income statement, we present subtotals of total operating profit, profit before tax and profit after tax from continuing operations, together with additional subtotals excluding exceptional items and remeasurements as a result of the three columnar layout described earlier. Exceptional items and remeasurements are presented in a separate column on the face of the income statement. • Customer contributions: contributions received prior to 1 July 2009 towards capital expenditure are recorded as deferred income and amortised in line with the depreciation on the associated asset. • Financial instruments: we normally opt to apply hedge accounting in most circumstances where this is permitted. For net investment hedges, we have chosen to use the spot rate method, rather than the alternative forward rate method. New IFRS accounting standards effective for the year ended 31 March 2018 The Group has adopted the following amendments to standards: • annual improvements to IFRSs 2014-2016 Cycle; • amendments to IAS 7 ‘Statement of cash flows’; and • amendments to IAS 12 ‘Income taxes’. The adoption of these amendments has had no material impact on the Group’s results or financial statement disclosures. 1. Basis of preparation and recent accounting developments continued New IFRS accounting standards and interpretations not yet adopted The Group enters into a significant number of transactions that fall within the scope of IFRS 9 ‘Financial Instruments’, IFRS 15 ‘Revenue from Contracts with Customers’ and IFRS 16 ‘Leases’. We are assessing the likely impact of these standards on the Group’s financial statements. i) IFRS 9 ‘Financial Instruments’ IFRS 9 ‘Financial Instruments’ is effective for National Grid for the year ending 31 March 2019. The change to IFRS 9 principally impacts the accounting for the classification and measurement of financial instruments, impairment of financial assets and hedge accounting. The Group has elected not to restate comparatives on initial application of IFRS 9. The full impact of adopting IFRS 9 will depend on the financial instruments that the Group has during the year ending 31 March 2019 as well as on economic conditions and judgements made as at the year end. The Group has performed an assessment of the potential impact of adopting IFRS 9 based on the financial instruments and hedging relationships as at the date of adoption of IFRS 9 (1 April 2018). It is not expected that the adoption of IFRS 9 will materially impact our profits or net assets on transition or prospectively. Classification and measurement: financial assets The number of categories of financial assets has been reduced under IFRS 9 compared to IAS 39. Under IFRS 9 the classification of financial assets is based on the business model within which the asset is held and the contractual terms of the asset. There are three principal classification categories for financial assets that are debt instruments: (i) amortised cost, (ii) fair value through other comprehensive income (FVOCI), and (iii) fair value through profit or loss (FVTPL). Equity investments are either classified as (i) FVTPL or (ii) FVOCI. Where assets or liabilities are measured at FVTPL, any fair value movements will be reported as remeasurements. If measured at FVOCI, realised gains on equity investments are not recycled to the income statement but instead are transferred directly to retained profits. The largest reallocation of financial assets will be from available-for-sale investments to FVTPL and relates to over £2 billion Group investments in money market funds and insurance company investments in mutual funds, where the contractual terms are such that they do not qualify for any other category. All other available-for-sale investments will be categorised as FVOCI under IFRS 9. The change to the asset classification rules will have no impact on reported Group net assets, although there will be some changes to reserves at transition. Classification and measurement: financial liabilities Under IFRS 9 financial liabilities can be designated at FVTPL to eliminate an accounting mismatch. Where financial liabilities are designated at FVTPL, changes in their fair value due to credit risk are presented in other comprehensive income. Remaining changes in fair value are presented in the income statement. With effect from 1 April 2018 the Group will take the fair value option for one issued zero coupon liability to reduce the measurement mismatch against some derivatives. This change will result in a reduction of reported net assets of less than £50 million . Impairment The impairment model under IFRS 9 reflects expected credit losses, as opposed to only incurred losses under IAS 39. The new impairment model will apply to the Group’s financial assets that are debt instruments measured at amortised cost or FVOCI as well as the Group’s trade receivables. The Group expects to apply the simplified approach, recognising lifetime expected losses for its trade receivables. The Group’s preliminary calculation of the loss allowance for these assets as at 31 March 2018 results in an immaterial impact compared to under IAS 39. The Group’s other investments in debt instruments that are subject to the IFRS 9 impairment model are determined to be low credit risk at 31 March 2018. The Group intends to apply the low credit risk simplification in IFRS 9, which allows the Group to assume that there has not been a significant increase in credit risk since initial recognition of these assets, and therefore recognise a loss allowance for only 12 -month expected credit losses as at 1 April 2018. The adjustment to the opening reserves in respect of this is not expected to be significant. Hedge accounting On initial application of IFRS 9, an entity may choose to continue to apply the hedge accounting requirements of IAS 39 instead of those of IFRS 9. The Group has elected to apply the IFRS 9 hedge accounting requirements because they more closely align with the Group’s risk management policies. An assessment of the Group’s designated hedging relationships under IAS 39 has been performed and it has been determined that all would qualify as continuing hedge relationships under IFRS 9. However, in order to apply elective changes to the treatment of costs of hedging, certain relationships will be formally redesignated from the date of adoption. The Group is considering additional opportunities to apply hedge accounting under IFRS 9. The Group does not anticipate the application of IFRS 9 hedge accounting requirements will have a material impact on the Group’s consolidated financial statements. 1. Basis of preparation and recent accounting developments continued ii) IFRS 15 ‘Revenue from Contracts with Customers’ IFRS 15 ‘Revenue from Contracts with Customers’ is effective for National Grid for the year ending 31 March 2019. The new standard provides enhanced detail and a five-step revenue recognition approach to reflect the transfer of goods and services to customers. The core principle of IFRS 15 is that an entity recognises revenue related to the transfer of promised goods or services when control of the goods or services passes to customers. The amount of revenue recognised should reflect the consideration to which the entity expects to be entitled in exchange for those goods or services. This differs from the principle under the current revenue standard that requires an assessment of when risks and rewards of goods and services are transferred rather than control of those goods or services. Detailed reviews of revenue arrangements in the UK and US have been undertaken prior to our transition to IFRS 15 on 1 April 2018. We will adopt the modified retrospective approach whereby the historical cumulative transition adjustment is reflected through retained earnings. There are two types of revenue arrangements that will be impacted on transition to IFRS 15. The financial impacts and the transition adjustment to retained earnings are described further: • There are certain pass-through revenues (principally revenues collected on behalf of the Scottish and Offshore transmission operators) where the principal/agency assessment changes on transition to IFRS 15. In moving from a risk and reward model to a control model, we will no longer record our revenues collected on behalf of the Scottish and Offshore transmission operators as principal as we do not control the Scottish or Offshore transmission networks. If we had adopted IFRS 15 in 2017/18, both revenues and operating costs would have been £1,056 million lower, with no impact to profit as a result of this change. There will be no transition adjustment as a result of this change; and • Across our subsidiaries in the UK and the US, our customers provide contributions for certain capital works (e.g. connections) which we continue to own on completion of the works. In our electricity business in the UK, we currently recognise customer contributions for connections over time as we have an ongoing contractual condition to maintain connections over their lives. In our UK Gas Transmission business, we recognise customer contributions when the connection is completed (the contractual conditions of the connection agreement do not explicitly require connections to be maintained over the life of the connection). In the US, revenue is also recognised when the connection is completed. Under IFRS 15, connection contributions in our subsidiaries will be deferred and released into the income statement as revenue over the life of the network. We have reached this conclusion because our customers cannot benefit from a connection without the use of our utility network; access to our network through the connection is satisfied over time. In the UK, we also have arrangements where our customers make contributions for diversions. These are currently deferred over the life of our network. Under IFRS 15, these revenues are recognised on completion of the diversion as there are no ongoing performance obligations to satisfy. Had we adopted IFRS 15 in 2017/18, revenues would have been approximately £83 million lower, as revenues from connections in the US and in UK Gas Transmission that were recognised up-front would have been deferred over the life of the network. The decrease in profit after tax in our subsidiaries would have been £56 million . The transition adjustment through retained earnings of £167 million will result in an increase to deferred revenues of approximately £240 million and a corresponding deferred tax impact of £73 million . iii) IFRS 16 ‘Leases’ IFRS 16 ‘Leases’ is effective for National Grid for the year ending 31 March 2020. The Group enters into a significant number of operating lease transactions. Under IFRS 16, our operating leases will be accounted for on the balance sheet as ‘right-of-use’ assets. This treatment will increase both our assets and liabilities and subsequently, will result in an increase in finance costs and depreciation and a reduction in operating costs. The outcome of our conclusions will have an impact on how we account for our operating leases. We are also performing an assessment of our revenue, service contracts and power purchase contracts to determine whether we have the right to use assets under those contracts and whether they fall within the scope of IFRS 16. We plan to apply IFRS 16 using the modified retrospective approach, whereby comparatives will not be restated on adoption of the new standard but instead a cumulative adjustment will be reflected in retained earnings. iv) Other In addition, the following new accounting standards and amendments to existing standards have been issued but are not yet effective or have not yet been endorsed by the EU: • Amendments to IFRS 2 ‘Share-based payment’; • IFRIC 22 ‘Foreign Currency Transactions and Advance Consideration’; • IFRIC 23 ‘Uncertainty over Income Tax Treatments’; • Amendments to IAS 40 ‘Investment Property’; • Amendments to IAS 28: 'Investments in associates' – Long-term interests in associates and joint ventures; • Annual Improvements to IFRS Standards 2015-2017 Cycle; • IFRS 17 ‘Insurance Contracts’; and • Amendments to IAS 19 ‘Employee Benefits’. Effective dates remain subject to the EU endorsement process. The Group is currently assessing the impact of the above standards, but they are not expected to have a material impact. The Group has not early adopted any other standard, amendment or interpretation that was issued but is not yet effective.

Segmental analysis

Segmental analysis	12 Months Ended
	Mar. 31, 2018
Operating Segments [Abstract]
Segmental analysis	Segmental analysis This note sets out the financial performance for the year split into the different parts of the business (operating segments). The performance of these operating segments is monitored and managed on a day-to-day basis. Revenue and the results of the business are analysed by operating segment, based on the information the Board of Directors uses internally for the purposes of evaluating the performance of operating segments and determining resource allocation between operating segments. The Board is National Grid’s chief operating decision maker (as defined by IFRS 8 ‘Operating Segments’) and assesses the profitability of operations principally on the basis of operating profit before exceptional items and remeasurements (see note 4). As a matter of course, the Board also considers profitability by segment, excluding the effect of timing. However, the measure of profit disclosed in this note is operating profit before exceptional items and remeasurements as this is the measure that is most consistent with the IFRS results reported within these financial statements. Our strategy in action The Group owns a portfolio of businesses that range from businesses with high levels of investment and growth to cash generative developed assets with lower investment requirements (such as National Grid Metering, included within NGV and Other). The majority of revenue is generated from regulated operating segments in the UK and US. The Group works with its regulators to obtain agreements that balance the risks faced with the opportunity to deliver reasonable returns for investors. When investing in NGV and Other, the Group aims to leverage its core capabilities to deliver higher returns for investors. The regulated businesses earn revenue for the transmission, distribution and generation services they have provided during the year. In any one year, the revenue recognised may differ from that allowed under the Group’s regulatory agreements and any such timing differences are adjusted through future prices. NGV and Other businesses earn revenue in line with their contractual terms. The following table describes the main activities for each reportable operating segment: UK Electricity Transmission High-voltage electricity transmission networks in England and Wales UK Gas Transmission High-pressure gas transmission networks in Great Britain and LNG storage activities US Regulated Gas distribution networks, electricity distribution networks and high-voltage electricity transmission networks in New York and New England and electricity generation facilities in New York NGV was formed on 1 April 2017 and brought together our businesses that are adjacent to our core regulated operations to create a new division with its own leadership. NGV is led by a member of the Group Executive Committee and its results are reported separately to the Board of Directors. This operating segment represents our key strategic growth area outside our regulated core business in competitive markets across the US and the UK. The business comprises all commercial operations in metering, LNG at the Isle of Grain in the UK and electricity interconnectors, with a focus on investment and future activities in emerging growth areas. NGV does not meet the thresholds set out in IFRS 8 to be identified as a separate reportable segment and therefore its results have not been disaggregated. The results of the businesses that now form NGV were previously reported in the Other activities segment and therefore, although the segment has been renamed NGV and Other, the results of previous periods have not been affected. Other activities that do not form part of any of the segments in the above table or NGV primarily relate to UK property development together with insurance and corporate activities in the UK and US. Discontinued operations in 2017 and 2016 comprise the profits and losses associated with the UK Gas Distribution business, up to and including the point at which it was sold to Quadgas HoldCo Limited (see note 9). In the current year, transactions within discontinued operations relate solely to the business prior to the sale and the sale transaction itself. Revenue primarily represents the sales value derived from the generation, transmission and distribution of energy, together with the sales value derived from the provision of other services to customers. It excludes value added (sales) tax and intra-group sales. Revenue includes an assessment of unbilled energy and transportation services supplied to customers between the date of the last meter reading and the year end. This is estimated based on historical consumption and weather patterns. Where revenue exceeds the maximum amount permitted by a regulatory agreement, adjustments will be made to future prices to reflect this over-recovery. No liability is recognised, as such an adjustment relates to the provision of future services. Similarly, no asset is recognised where a regulatory agreement permits adjustments to be made to future prices in respect of an under-recovery. As part of our regulatory agreements we are entitled to recover certain costs directly from customers (pass-through costs). These amounts are included in the overall calculation of allowed revenue as stipulated by regulatory agreements and explained further in the unaudited commentary on pages 186–191. Sales between operating segments are priced considering the regulatory and legal requirements to which the businesses are subject. The analysis of revenue by geographical area is on the basis of destination. There are no material sales between the UK and US geographical areas. 2. Segmental analysis continued (a) Revenue 2018 2017 2016 Total £m Sales between segments £m Sales to third parties £m Total Sales Sales Total Sales Sales Operating segments – continuing operations: UK Electricity Transmission 4,154 (28 ) 4,126 4,439 (29) 4,410 3,977 (20) 3,957 UK Gas Transmission 1,091 (9 ) 1,082 1,080 (99) 981 1,047 (109) 938 US Regulated 9,272 — 9,272 8,931 — 8,931 7,493 — 7,493 NGV and Other 1 776 (6 ) 770 713 — 713 824 — 824 Total revenue from continuing operations 15,293 (43 ) 15,250 15,163 (128) 15,035 13,341 (129) 13,212 Split by geographical areas – continuing operations: UK 5,938 6,064 5,619 US 9,312 8,971 7,593 15,250 15,035 13,212 1. Included within NGV and Other is £593 million ( 2017 : £604 million ; 2016 : £719 million ) of revenue relating to NGV. (b) Operating profit A reconciliation of the operating segments’ measure of profit to profit before tax from continuing operations is provided below. Further details of the exceptional items and remeasurements are provided in note 4. Before exceptional items After exceptional items 2018 2017 2016 2018 2017 2016 £m £m £m £m £m £m Operating segments – continuing operations: UK Electricity Transmission 1,041 1,372 1,173 1,041 1,361 1,173 UK Gas Transmission 487 511 486 487 507 486 US Regulated 1,698 1,713 1,185 1,734 1,278 1,196 NGV and Other 1 231 177 370 231 62 370 Total operating profit from continuing operations 3,457 3,773 3,214 3,493 3,208 3,225 Split by geographical area – continuing operations: UK 1,840 2,118 2,007 1,840 1,988 2,007 US 1,617 1,655 1,207 1,653 1,220 1,218 3,457 3,773 3,214 3,493 3,208 3,225 Below we reconcile total operating profit from continuing operations to profit before tax from continuing operations. We have shown the share of post-tax results of joint ventures and associates disaggregated between those held within NGV and Other and our retained 39% interest in the UK Gas Distribution business (Cadent 2 ). Operating exceptional items and remeasurements of £nil ( 2017 : £11 million cost; 2016 : £nil ) detailed in note 4 are attributable to UK Electricity Transmission; £nil ( 2017 : £4 million cost; 2016 : £nil ) to UK Gas Transmission; £36 million gain ( 2017 : £435 million cost; 2016 : £11 million gain) to US Regulated; and £nil ( 2017 : £115 million cost; 2016 : £nil ) to NGV and Other. Reconciliation to profit before tax: Operating profit from continuing operations 3,457 3,773 3,214 3,493 3,208 3,225 Finance income 154 53 22 154 53 22 Finance costs (1,128 ) (1,082 ) (878 ) (899 ) (1,140 ) (977 ) Share of post-tax results of joint ventures and associates: Cadent 2 123 — — (89 ) — — NGV and Other 44 63 59 49 63 59 Profit before tax from continuing operations 2,650 2,807 2,417 2,708 2,184 2,329 1. Included within NGV and Other is £234 million ( 2017 : £239 million ; 2016 : £394 million ) of operating profit (both before and after exceptional items and remeasurements) relating to NGV. 2. Investment held through Quadgas HoldCo Limited. 2. Segmental analysis continued (c) Capital expenditure Net book value of property, plant and Capital expenditure 1 Depreciation and amortisation 2018 2017 2016 2018 2017 2016 2018 2017 2016 £m £m £m £m £m £m £m £m £m Operating segments: UK Electricity Transmission 13,028 12,515 11,907 999 1,027 1,084 (475 ) (421 ) (390 ) UK Gas Transmission 4,280 4,165 4,140 310 214 186 (194 ) (186 ) (178 ) US Regulated 20,953 21,638 17,490 2,424 2,247 1,856 (635 ) (642 ) (535 ) NGV and Other 2 2,491 2,430 2,291 341 247 201 (226 ) (232 ) (208 ) Total from continuing operations 40,752 40,748 35,828 4,074 3,735 3,327 (1,530 ) (1,481 ) (1,311 ) Split by geographical area – continuing operations: UK 18,772 18,102 17,491 1,527 1,357 1,386 (804 ) (753 ) (715 ) US 21,980 22,646 18,337 2,547 2,378 1,941 (726 ) (728 ) (596 ) 40,752 40,748 35,828 4,074 3,735 3,327 (1,530 ) (1,481 ) (1,311 ) Asset type: Property, plant and equipment 39,853 39,825 35,074 3,901 3,507 3,130 (1,392 ) (1,348 ) (1,207 ) Non-current intangible assets 899 923 754 173 228 197 (138 ) (133 ) (104 ) Total from continuing operations 40,752 40,748 35,828 4,074 3,735 3,327 (1,530 ) (1,481 ) (1,311 ) 1. Represents additions to property, plant and equipment and non-current intangibles but excludes additional investments in and loans to joint ventures and associates. 2. Included within NGV and Other are assets with a net book value of £1,454 million ( 2017 : £1,432 million ; 2016 : £1,482 million ), capital expenditure of £186 million ( 2017 : £98 million ; 2016 : £93 million ) and depreciation and amortisation of £143 million ( 2017 : £143 million ; 2016 : £144 million ) relating to NGV. Total non-current assets other than financial instruments and pension assets located in the UK and US were £20,816 million and £27,663 million respectively as at 31 March 2018 ( 31 March 2017 : UK £20,045 million , US £28,951 million ; 31 March 2016 : UK £26,261 million , US £23,774 million ). Unaudited commentary on the results of our principal operations by segment – continuing operations Commentary on segmental adjusted operating profit results Below we summarise the results of our operating segments. This analysis has been prepared based on adjusted operating profit (operating profit before exceptional items and remeasurements) as set out in note 2(b). UK Electricity Transmission For the year ended 31 March 2018 , revenue in the UK Electricity Transmission segment decreased by £285 million to £4,154 million and adjusted operating profit decreased by £331 million to £1,041 million . The revenue reduction of £285 million included a reduction in pass-through costs such as system balancing costs charged on to customers. Excluding pass-through costs, net revenue was £235 million lower, reflecting the absence of last year’s recovery of outstanding timing balances along with higher adjustments this year to return the benefits of efficiencies and lower required outputs to customers. Regulated controllable costs were £35 million higher reflecting inflation, increased headcount and workload and initiative spend. Depreciation and amortisation was £54 million higher, reflecting the continued capital investment programme. Other costs were in line with the prior year. Capital expenditure decreased by £28 million compared with last year to £999 million . UK Gas Transmission Revenue in the UK Gas Transmission segment increased by £11 million to £1,091 million and adjusted operating profit decreased by £24 million to £487 million . After deducting pass-through costs, net revenue was £23 million lower than prior year. Increases in allowed revenues this year were more than offset by the end of certain legacy revenue allowances and the refund of prior year over-recoveries to customers. Regulated controllable costs were £9 million higher than last year, mainly as a result of higher employee numbers to deliver additional outputs. Depreciation and amortisation costs were £8 million higher, reflecting ongoing investment. Other operating costs were £16 million lower than last year, including the release of unused provisions relating to LNG plant closures. Capital expenditure increased to £310 million , £96 million higher than last year, with increases in asset health spend and higher investment on compressor projects. US Regulated Revenue in our US Regulated business increased by £341 million to £9,272 million and adjusted operating profit decreased by £15 million to £1,698 million . The weaker US dollar decreased revenue and operating profit in the year by £534 million and £102 million respectively. Excluding the impact of foreign exchange rate movements, revenue increased by £875 million . Of this increase, £597 million was due to increases in pass-through costs charged on to customers. Excluding pass-through costs, net revenue increased by £278 million at constant currency, reflecting increased revenue allowances under new rate plans in downstate New York and Massachusetts Electric, and the benefit of capital trackers. We have incurred £142 million of major storm costs in 2017/18 including a sequence of heavy storms this winter causing substantial damage to our electricity networks. Separate from these costs, regulated controllable costs were broadly in line with last year at constant currency, and bad debt costs were £13 million lower. Depreciation and amortisation was £31 million higher this year at constant currency as a result of ongoing investment in our networks. Other operating costs were £34 million higher at constant currency, reflecting higher property taxes. Capital expenditure in the US Regulated business increased to £2,424 million this year, £177 million more than in 2016/17. At constant currency, this represented a £311 million increase in investment driven by higher investment in new and replacement gas mains. NGV and Other Revenue in NGV and Other increased by £63 million to £776 million and adjusted operating profit increased by £54 million to £231 million . This reflects higher revenues and profit on disposal of property sites in the UK and lower levels of business change costs incurred, partially offset by lower auction revenues in the French Interconnector. Capital expenditure in NGV and Other was £94 million higher than last year at £341 million , including the start of construction of a second French Interconnector and increases in smart meter installations in the UK.

Operating costs

Operating costs	12 Months Ended
	Mar. 31, 2018
Analysis of income and expense [abstract]
Operating costs	Operating costs Below we have presented separately certain items included in our operating costs from continuing operations. These include a breakdown of payroll costs (including disclosure of amounts paid to key management personnel) and fees paid to our auditors. Rentals under operating leases are charged to the income statement on a straight-line basis over the term of the relevant lease. Before exceptional items and remeasurements Exceptional items and remeasurements Total 2018 2017 2016 2018 2017 2016 2018 2017 2016 £m £m £m £m £m £m £m £m £m Depreciation and amortisation 1,530 1,481 1,311 — — — 1,530 1,481 1,311 Payroll costs 1,648 1,578 1,337 — — — 1,648 1,578 1,337 Purchases of electricity 1,299 1,143 1,304 (14 ) (46 ) 8 1,285 1,097 1,312 Purchases of gas 1,539 1,241 986 4 (22 ) (19 ) 1,543 1,219 967 Rates and property taxes 1,057 1,042 899 — — — 1,057 1,042 899 Balancing Services Incentive Scheme 1,012 1,120 907 — — — 1,012 1,120 907 Payments to other UK network owners 1,043 1,008 971 — — — 1,043 1,008 971 Other 2,665 2,649 2,283 (26 ) 633 — 2,639 3,282 2,283 11,793 11,262 9,998 (36 ) 565 (11 ) 11,757 11,827 9,987 Operating costs include: Inventory consumed 367 296 274 Operating leases 102 98 91 Research and development expenditure 13 14 19 (a) Payroll costs 2018 2017 2016 £m £m £m Wages and salaries 1 1,998 1,852 1,553 Social security costs 157 145 120 Defined contribution scheme costs 65 58 47 Defined benefit pension costs (see note 23) 156 151 154 Share-based payments 16 32 21 Severance costs (excluding pension costs) 7 5 4 2,399 2,243 1,899 Less: payroll costs capitalised (751 ) (665 ) (562 ) Total payroll costs 1,648 1,578 1,337 1. Included within wages and salaries are US other post-retirement benefit costs of £46 million ( 2017 : £53 million ; 2016 : £52 million ). For further information refer to note 23. (b) Number of employees 31 March 2018 Monthly 31 March 2017 Monthly 31 March 2016 Monthly UK 6,517 6,431 6,265 6,291 6,224 6,067 US 16,506 16,274 15,867 15,752 14,830 14,775 Total number of employees 23,023 22,705 22,132 22,043 21,054 20,842 3. Operating costs continued (c) Key management compensation 2018 2017 2016 £m £m £m Short-term employee benefits 8 8 9 Post-employment benefits 1 1 1 Share-based payments 3 6 4 Total key management compensation 12 15 14 Key management compensation relates to the Board, including the Executive Directors and Non-executive Directors for the years presented. (d) Directors’ emoluments Details of Executive Directors’ emoluments are contained in the Remuneration Report on page 70 and those of Non-executive Directors on page 75. (e) Auditors’ remuneration Auditors’ remuneration is presented below in accordance with the requirements of the Companies Act 2006 and the principal accountant fees and services disclosure requirements of Item 16C of Form 20-F: 2018 2017 2016 £m £m £m Audit fees payable to the parent Company’s auditors 1 and their associates in respect of: Audit of the parent Company’s individual and consolidated financial statements 2 2.7 1.5 1.3 The auditing of accounts of any associate of the Company 7.5 13.7 9.2 Other services supplied 3 3.9 4.6 3.6 14.1 19.8 14.1 Total other services 4 Tax fees: Tax compliance services 0.3 0.4 0.5 Tax advisory services — 0.1 — All other fees: Other assurance services 5 0.7 4.6 4.3 Services relating to corporate finance transactions not covered above 6 — 5.9 1.6 Other non-audit services not covered above 7 0.9 6.3 2.5 1.9 17.3 8.9 Total auditors’ remuneration 16.0 37.1 23.0 1. Deloitte LLP became the Group’s principal auditor for the year ended 31 March 2018 . PricewaterhouseCoopers LLP (PwC) was the principal auditor for the years ended 31 March 2017 and 31 March 2016 . 2. Audit fees in each year represent fees for the audit of the Company’s financial statements and regulatory reporting for the years ended 31 March 2018 , 2017 and 2016 . 3. Other services supplied represent fees payable for services in relation to other statutory filings or engagements that are required to be carried out by the auditors. In particular, this includes fees for reports under section 404 of the US Public Company Accounting Reform and Investor Protection Act of 2002 (Sarbanes-Oxley), audit reports on regulatory returns and the review of interim financial statements for the six-month periods ended 30 September 2017, 2016 and 2015 respectively. 4. There were no audit related fees as described in Item 16C(b) of Form 20-F. 5. Principally amounts relating to assurance services provided in relation to comfort letters for debt issuances. In 2016/17, amounts represented assurance services undertaken by PwC in relation to the sale of UK Gas Distribution and data assurance work in respect of financial information included in US rate filings. 6. Vendor due diligence and other transaction services in relation to the sale of UK Gas Distribution. 7. Fees for other non-audit services – projects including services provided to the UK Property business, relating to evaluating possible options for the use of property assets. In 2016/17, services related principally to PwC assisting the Company with separation activities in relation to the sale of UK Gas Distribution. For the year ended 31 March 2017 , PwC contracted with Ofgem to assess the UK gas industry’s readiness for the introduction of new settlement processes and systems. Fees for these services were paid by Xoserve Limited, a subsidiary of National Grid (until 31 March 2017 ), on behalf of the industry, under instruction from Ofgem. As PwC had no contract with or duty of care to Xoserve Limited, these amounts were not included above. The Audit Committee considers and makes recommendations to the Board, to be put to shareholders for approval at each AGM, in relation to the appointment, re-appointment, removal and oversight of the Company’s independent auditors. The Board of Directors, in accordance with a resolution approved at the 2016 AGM, is authorised to agree the auditors’ remuneration. The Audit Committee considers and approves the audit fees on behalf of the Board in accordance with the Competition and Market Authority Audit Order 2014. The Board of Directors will seek to renew this authority at the 2018 AGM. Details of our policies and procedures in relation to non-audit services to be provided by the independent auditors are set out within page 55 of the Corporate Governance Report. Certain services are prohibited from being performed by the external auditors under the Sarbanes-Oxley Act. Of the above services, none were prohibited.

Exceptional items and remeasure

Exceptional items and remeasurements	12 Months Ended
	Mar. 31, 2018
Exceptional Items And Remeasurements [Abstract]
Exceptional items and remeasurements	Exceptional items and remeasurements To monitor our financial performance, we use a profit measure that excludes certain income and expenses. We call that measure ‘business performance’ or ‘adjusted profit’. We exclude items from business performance because, if included, these items could distort understanding of our performance for the year and the comparability between periods. This note analyses these items, which are included in our results for the year but are excluded from business performance. Our financial performance is analysed into two components: business performance, which excludes exceptional items and remeasurements; and exceptional items and remeasurements. Business performance is used by management to monitor financial performance as it is considered that it improves the comparability of our reported financial performance from year to year. Business performance sub-totals are presented on the face of the income statement or in the notes to the financial statements. Management utilises an exceptional items framework that has been discussed and approved by the Audit Committee. This follows a three-step process which considers the nature of the event, the financial materiality involved and any particular facts and circumstances. In considering the nature of the event, management focuses on whether the event is within the Group’s control and how frequently such an event typically occurs. In determining the facts and circumstances management considers factors such as ensuring consistent treatment between favourable and unfavourable transactions, precedent for similar items, number of periods over which costs will be spread or gains earned and the commercial context for the particular transaction. Items of income or expense that are considered by management for designation as exceptional items include significant restructurings, write-downs or impairments of non-current assets, significant changes in environmental or decommissioning provisions, integration of acquired businesses, gains or losses on disposals of businesses or investments and significant debt redemption costs as a consequence of transactions such as significant disposals or issues of equity, and the related tax as well as deferred tax arising on changes to corporation tax rates. Costs arising from restructuring programmes include redundancy costs. Redundancy costs are charged to the income statement in the year in which a commitment is made to incur the costs and the main features of the restructuring plan have been announced to affected employees. Remeasurements comprise gains or losses recorded in the income statement arising from changes in the fair value of commodity contract derivatives and of derivative financial instruments to the extent that hedge accounting is not achieved or is not effective. These fair values increase or decrease because of changes in commodity and financial indices and prices over which we have no control. Exceptional items and remeasurements from continuing operations 2018 2017 2016 £m £m £m Included within operating profit Exceptional items: Environmental charges — (526 ) — Gas holder demolition costs — (107 ) — Final settlement of LIPA MSA Transition 26 — — 26 (633 ) — Remeasurements – commodity contract derivatives 10 68 11 36 (565 ) 11 Included within finance costs Remeasurements – net gains/(losses) on derivative financial instruments 229 (58 ) (99 ) Included within share of post-tax results of joint ventures and associates Remeasurements – net gains on financial instruments 1 — — Exceptional items: Deferred tax arising on the reduction in US corporation tax rate 5 — — Impairment of investment in Quadgas HoldCo Limited (213 ) — — (207 ) — — Total included within profit before tax 58 (623 ) (88 ) Included within tax Exceptional items – credits arising on items not included in profit before tax: Deferred tax arising on the reduction in the UK corporation tax rate — 94 162 Deferred tax arising on the reduction in the US corporation tax rate 1,510 — — Tax on exceptional items (9 ) 227 — Tax on remeasurements (28 ) (29 ) 15 1,473 292 177 Total exceptional items and remeasurements after tax 1,531 (331 ) 89 Analysis of total exceptional items and remeasurements after tax Exceptional items after tax 1,319 (312 ) 162 Remeasurements after tax 212 (19 ) (73 ) Total exceptional items and remeasurements after tax 1,531 (331 ) 89 4. Exceptional items and remeasurements continued Operating exceptional items 2017/18 During the year, the Group reached an agreement with LIPA on an amount in final settlement of receivables and payables that arose following the cessation of the Management Services Agreement with LIPA in December 2013. The settlement has resulted in a gain of £26 million , which has been recorded as exceptional, consistent with the treatment of gains and losses on the original transaction. In assessing the value of the Group’s interests in Quadgas HoldCo Limited (the holding company for Cadent Gas) at 31 March 2018, the Company has considered the fair market value of its interests as implied by the agreement relating to the potential sale of a 25% interest in Quadgas HoldCo Limited (equity and shareholder loans), announced on 1 May 2018 and described in note 15. The associated accounting implications are the recognition of a £110 million fair value gain on the Further Acquisition Agreement (FAA) which is detailed in the remeasurements section below and a £213 million impairment against the equity carrying value of investment in Quadgas HoldCo Limited. We have assessed the carrying value of all our interests in Quadgas HoldCo Limited (including the FAA derivative asset noted above) against the cash flows we expect to receive under the agreement for the 25% (comprising future dividends, shareholder loan interest income and the proceeds on exercise of the option arrangement plus a cost of carry), discounted to present value using an estimate of Quadgas Investments BidCo Limited’s marginal cost of borrowing. Following the recognition of this charge, the total carrying value of our interests in Quadgas HoldCo Limited is £2.1 billion . Neither of these two accounting entries are taxable. 2016/17 In the US, the Group’s most significant environmental liabilities relate to former manufacturing gas plant (MGP) facilities formerly owned or operated by the Group. The sites are subject to both state and federal law in the US. Environmental reserves are re-evaluated at each reporting period. The expenditure is expected to be largely recoverable from rate payers but, under IFRS, no asset can be recorded for this. During the second half of 2016/17, the Group updated its assessment of the gross remediation costs at three key sites in New York, resulting in an increase of £481 million on an undiscounted basis. The charge booked reflects the Group’s best estimate of future cash outflow, based on notices received from state and federal authorities, and plans developed in response, supported by external consultants where appropriate. In some cases, judgement is also required regarding the Group’s share of the estimated cost, principally at sites where other parties are also potentially liable but where no cost sharing agreement exists. Also included within the above are charges relating to the impact of a change in the real discount rate from 2% to 1% on our provisions. A provision of £107 million was made for the demolition of certain non-operational gas holders in the UK. Following the disposal of UK Gas Distribution, the land on which the gas holders are sited was transferred to the Group’s UK property division. The Group’s property division maximises our return from our land portfolio and therefore a constructive obligation exists to demolish the gas holders. Remeasurements As described further in note 30(f), the FAA signed on 31 March 2017 relating to a 14% interest in the equity and shareholder loans of Quadgas HoldCo Limited is treated as a derivative at fair value through profit and loss. In assessing the fair value of this derivative at 31 March 2018, we have compared the pricing mechanism within the FAA against that of the agreement concerning our remaining 25% interest. The £110 million gain reflects the pricing differential between the two contracts. At 31 March 2017, being the date on which the FAA was signed, the fair value was taken to be zero . Commodity contract derivatives represent mark-to-market movements on certain physical and financial commodity contract obligations in the US. These contracts primarily relate to the forward purchase of energy for supply to customers, or to the economic hedging thereof, that are required to be measured at fair value and that do not qualify for hedge accounting. Under the existing rate plans in the US, commodity costs are recoverable from customers although the timing of recovery may differ from the pattern of costs incurred. Net losses or gains on derivative financial instruments comprise losses or gains arising on derivative financial instruments reported in the income statement. These exclude gains and losses for which hedge accounting has been effective, which have been recognised directly in other comprehensive income or which are offset by adjustments to the carrying value of debt. Therefore, these are always excluded from business performance. Net gains on financial instruments comprise the gains on financial instruments of Quadgas HoldCo Limited reported through their income statement. Items included within tax The Tax Cuts and Jobs Act (Tax Reform) which was enacted on 22 December 2017 reduced the US corporate rate from 35% to 21% with effect from 1 January 2018. Deferred taxes at the reporting date have been measured using these enacted tax rates. Since we are in a net deferred tax liability position in the US, this results in a deferred tax credit in the year. As described further in note 10, however, we expect the overall impact of Tax Reform to be economically neutral for the Group. The Finance Act 2016 which was enacted on 15 September 2016 reduced the main rate of UK corporation tax to 17% with effect from 1 April 2020. Deferred tax balances have been calculated at this rate for the years ended 31 March 2017 and 31 March 2018. The Finance Act 2015 (No. 2) was enacted on 18 November 2015 which reduced the main rate of UK corporation tax to 19% with effect from 1 April 2017 and 18% from 1 April 2020. Deferred tax balances were calculated at 18% for the year ended 31 March 2016.

Finance income and costs

Finance income and costs	12 Months Ended
	Mar. 31, 2018
Analysis of income and expense [abstract]
Finance income and costs	Finance income and costs This note details the interest income generated by our financial assets and interest expense incurred on our financial liabilities. It also includes the net interest on our pensions and other post-retirement assets. In reporting business performance, we adjust net financing costs to exclude any net gains or losses on derivative financial instruments included in remeasurements. In addition, significant debt redemption costs are typically treated as exceptional (see note 4). 2018 2017 2016 Notes £m £m £m Finance income Interest income on financial instruments: Bank deposits and other financial assets 81 28 22 Gains on disposal of available-for-sale investments 73 25 — 154 53 22 Finance costs Net interest on pensions and other post-retirement benefit obligations 23 (65 ) (107 ) (111 ) Interest expense on financial liabilities held at amortised cost: Bank loans and overdrafts (87 ) (59 ) (28 ) Other borrowings (1,030 ) (927 ) (792 ) Derivatives 12 (8 ) 37 Unwinding of discount on provisions 24 (75 ) (73 ) (69 ) Other interest (11 ) (17 ) (27 ) Less: interest capitalised 1 128 109 112 (1,128 ) (1,082 ) (878 ) Remeasurements Net gains/(losses) on derivative financial instruments included in remeasurements 2 : Ineffectiveness on derivatives designated as: Fair value hedges 3 34 33 39 Cash flow hedges 10 (12 ) (15 ) Net investment hedges – undesignated forward rate risk 5 60 (34 ) Derivatives not designated as hedges or ineligible for hedge accounting 4 180 (139 ) (89 ) 229 (58 ) (99 ) (899 ) (1,140 ) (977 ) Net finance costs from continuing operations (745 ) (1,087 ) (955 ) 1. Interest on funding attributable to assets in the course of construction in the current year was capitalised at a rate of 4.1% ( 2017 : 3.4% ; 2016 : 3.3% ). In the UK, capitalised interest qualifies for a current year tax deduction with tax relief claimed of £20 million ( 2017 : £18 million ; 2016 : £19 million ). In the US, capitalised interest is added to the cost of plant and qualifies for tax depreciation allowances. 2. Includes a net foreign exchange loss on financing activities of £314 million ( 2017 : £264 million loss; 2016 : £407 million loss) offset by foreign exchange gains and losses on derivative financial instruments measured at fair value. 3. Includes a net loss on instruments designated as fair value hedges of £90 million ( 2017 : £27 million loss; 2016 : £34 million gain) and a net gain of £124 million ( 2017 : £60 million gain; 2016 : £5 million gain) arising from fair value adjustments to the carrying value of debt. 4. Includes £110 million gain on the Further Acquisition Agreement (FAA) derivative financial instrument relating to the put/call option over a 14% interest in Quadgas HoldCo Limited. Further details can be found in note 15.

Tax

Tax	12 Months Ended
	Mar. 31, 2018
Income Taxes [Abstract]
Tax	Tax Tax is payable in the territories where we operate, mainly the UK and the US. This note gives further details of the total tax charge and tax liabilities, including current and deferred tax. The current tax charge is the tax payable on this year’s taxable profits. Deferred tax is an accounting adjustment to provide for tax that is expected to arise in the future due to differences in the accounting and tax bases. The tax charge for the period is recognised in the income statement, the statement of comprehensive income or directly in equity, according to the accounting treatment of the related transaction. The tax charge comprises both current and deferred tax. Current tax assets and liabilities are measured at the amounts expected to be recovered from or paid to the tax authorities. The tax rates and tax laws used to compute the amounts are those that have been enacted or substantively enacted by the reporting date. The Group operates internationally in territories with different and complex tax codes. Management exercises judgement in relation to the level of provision required for uncertain tax outcomes. There are a number of tax positions not yet agreed with the tax authorities where different interpretations of legislation could lead to a range of outcomes. Judgements are made for each position having regard to particular circumstances and advice obtained. Deferred tax is provided for using the balance sheet liability method and is recognised on temporary differences between the carrying amount of assets and liabilities in the financial statements and the corresponding tax bases. Deferred tax liabilities are generally recognised on all taxable temporary differences and deferred tax assets are recognised to the extent that it is probable that taxable profits will be available against which deductible temporary differences can be utilised. However, deferred tax assets and liabilities are not recognised if the temporary differences arise from the initial recognition of goodwill or from the initial recognition of other assets and liabilities in a transaction (other than a business combination) that affects neither the accounting nor the taxable profit or loss. Deferred tax liabilities are recognised on taxable temporary differences arising on investments in subsidiaries and joint arrangements except where the Company is able to control the reversal of the temporary difference and it is probable that the temporary difference will not reverse in the foreseeable future. Deferred tax is calculated at the tax rates that are expected to apply in the period when the liability is settled or the asset is realised, based on the tax rates and tax laws that have been enacted or substantively enacted by the reporting date. The carrying amount of deferred tax assets is reviewed at each reporting date and reduced to the extent that it is no longer probable that sufficient taxable profits will be available to allow all or part of the deferred tax asset to be recovered. Unrecognised deferred tax assets are reassessed at each reporting date and are recognised to the extent that it has become probable that future taxable profits will allow the deferred tax asset to be recovered. Deferred tax assets and liabilities are offset when there is a legally enforceable right to offset current tax assets against current tax liabilities and when they relate to income taxes levied by the same tax authority and the Company and its subsidiaries intend to settle their current tax assets and liabilities on a net basis. Tax (credited)/charged to the income statement – continuing operations 2018 2017 2016 £m £m £m Tax before exceptional items and remeasurements 589 666 604 Exceptional tax on items not included in profit before tax (see note 4) (1,510 ) (94 ) (162 ) Tax on other exceptional items and remeasurements 37 (198 ) (15 ) Tax on total exceptional items and remeasurements (1,473 ) (292 ) (177 ) Total tax (credit)/charge from continuing operations (884 ) 374 427 Tax as a percentage of profit before tax 2018 2017 2016 % % % Before exceptional items and remeasurements – continuing operations 22.2 23.7 25.0 After exceptional items and remeasurements – continuing operations (32.6 ) 17.1 18.3 6. Tax continued The tax credit for the year can be analysed as follows: 2018 2017 2016 £m £m £m Current tax: UK corporation tax at 19% (2017: 20%; 2016: 20%) 205 225 239 UK corporation tax adjustment in respect of prior years (18 ) (47 ) (5 ) 187 178 234 Overseas corporation tax 15 — 38 Overseas corporation tax adjustment in respect of prior years (4 ) 1 (19 ) 11 1 19 Total current tax from continuing operations 198 179 253 Deferred tax: UK deferred tax 65 (9 ) (80 ) UK deferred tax adjustment in respect of prior years (2 ) (18 ) 24 63 (27 ) (56 ) Overseas deferred tax (1,155 ) 224 229 Overseas deferred tax adjustment in respect of prior years 10 (2 ) 1 (1,145 ) 222 230 Total deferred tax from continuing operations (1,082 ) 195 174 Total tax (credit)/charge from continuing operations (884 ) 374 427 Tax charged/(credited) to other comprehensive income and equity 2018 2017 2016 £m £m £m Current tax: Share-based payments (3 ) (4 ) (1 ) Available-for-sale investments (11 ) 6 5 Deferred tax: Available-for-sale investments (18 ) 8 12 Cash flow hedges (4 ) 20 22 Share-based payments 1 1 — Remeasurements of gains of pension assets and post-retirement benefit obligations 1 530 277 95 495 308 133 Total tax recognised in the statements of comprehensive income from continuing operations 497 311 134 Total tax recognised in the statements of comprehensive income from discontinued operations — 10 23 Total tax relating to share-based payments recognised directly in equity from continuing operations (2 ) (3 ) (1 ) Total tax relating to share-based payments recognised directly in equity from discontinued operations — — (1 ) 495 318 155 1. Remeasurements of gains of pension assets and post-retirement benefit obligations includes a deferred tax charge of £281 million arising on the reduction in the US corporation tax rate. 6. Tax continued The tax credit for the year after exceptional items and remeasurements, for the continuing business, is lower ( 2017 : lower tax charge; 2016 : lower tax charge) than the standard rate of corporation tax in the UK of 19% ( 2017 : 20% ; 2016 : 20% ): Before exceptional items and remeasurements After exceptional items and remeasurements Before exceptional items and remeasurements After exceptional items and remeasurements Before exceptional items and remeasurements After exceptional items and remeasurements 2018 2018 2017 2017 2016 2016 £m £m £m £m £m £m Profit before tax from continuing operations Before exceptional items and remeasurements 2,650 2,650 2,807 2,807 2,417 2,417 Exceptional items and remeasurements — 58 — (623 ) — (88 ) Profit before tax from continuing operations 2,650 2,708 2,807 2,184 2,417 2,329 Profit before tax from continuing operations multiplied by UK corporation tax rate of 19% (2017: 20%; 2016: 20%) 503 515 561 437 483 465 Effect of: Adjustments in respect of prior years 1 (22 ) (14 ) (67 ) (67 ) 2 1 Expenses not deductible for tax purposes 2 20 21 35 442 25 114 Non-taxable income 2 (16 ) (47 ) (24 ) (425 ) (25 ) (112 ) Adjustment in respect of foreign tax rates 153 157 180 104 124 129 Deferred tax impact of change in UK tax rate (7 ) (7 ) — (94 ) — (162 ) Deferred tax impact of change in US tax rate due to Tax Reform — (1,510 ) — — — — Other 3 (42 ) 1 (19 ) (23 ) (5 ) (8 ) Total tax charge/(credit) from continuing operations 589 (884 ) 666 374 604 427 % % % % % % Effective tax rate – continuing operations 22.2 (32.6 ) 23.7 17.1 25.0 18.3 1. Prior year adjustment is primarily due to agreement of prior period tax returns. 2. For the years ended 31 March 2017 and prior, the adjustments after exceptional items and remeasurements primarily represent the impact of the Group’s net investment hedging following significant US dollar currency fluctuations. 3. Other primarily comprises tax on joint ventures and associates. Factors that may affect future tax charges The Finance Act 2016 which was enacted on 15 September 2016 reduced the main rate of UK corporation tax to 17% with effect from 1 April 2020. Deferred tax balances have been calculated at this rate. We will continue to monitor the developments driven by Brexit, the OECD’s Base Erosion and Profit Shifting (BEPS) project and European Commission initiatives including fiscal state aid investigations. At this time we do not expect this to cause any material impact on our future tax charges. On 22 December 2017, the Tax Cuts and Jobs Act (Tax Reform) was signed into law in the US. The Tax Reform includes significant changes to various federal tax provisions applicable to National Grid. The most significant changes include the reduction in the corporate federal income tax rate from 35% to 21% effective 1 January 2018 and the elimination of bonus depreciation deduction on utility property, plant and equipment acquired after 27 September 2017 but allowance for the 100% expensing of non-utility property, plant and equipment. The reduction in the US corporate tax rate is the only item we would expect to materially impact our future effective tax rate. 6. Tax continued Tax included within the statement of financial position The following are the major deferred tax assets and liabilities recognised, and the movements thereon, during the current and prior reporting periods: Accelerated Share- Pensions Financial Other net 1 Total £m £m £m £m £m £m Deferred tax liabilities/(assets) At 1 April 2016 7,063 (14 ) (1,038 ) (53 ) (1,324 ) 4,634 Exchange adjustments and other 2 681 1 (144 ) (7 ) (50 ) 481 Charged/(credited) to income statement 402 — 177 23 (481 ) 121 Charged to other comprehensive income and equity — 1 264 46 5 316 Disposal of UK Gas Distribution (1,072 ) — (6 ) — 5 (1,073 ) At 1 April 2017 7,074 (12 ) (747 ) 9 (1,845 ) 4,479 Exchange adjustments and other 2 (559 ) — 69 1 221 (268 ) (Credited)/charged to income statement (1,641 ) 2 (55 ) 12 598 (1,084 ) Charged/(credited) to other comprehensive income and equity — 1 530 (1 ) (21 ) 509 At 31 March 2018 4,874 (9 ) (203 ) 21 (1,047 ) 3,636 1. The deferred tax asset of £1,047 million as at 31 March 2018 in respect of other net temporary differences primarily relates to US net operating losses ( £390 million ) and environmental provisions ( £378 million ). 2. Exchange adjustments and other comprises of foreign exchange arising on translation of the US dollar deferred tax balances together with a reclassification of £43 million (2017: £143 million ) being the opening deferred tax balance in respect of US net operating losses to offset against US current tax liabilities. Deferred tax assets and liabilities are only offset where there is a legally enforceable right of offset and there is an intention to settle the balances net. The deferred tax balances (after offset) for statement of financial position purposes consist solely of deferred tax liabilities of £3,636 million ( 2017 : £4,479 million ). This balance is after offset of a deferred tax asset of £390 million ( 2017 : £798 million ) which has been recognised in respect of net operating losses. The deferred tax credited to the income statement of £1,084 million is after offset of a £293 million deferred tax charge in respect of net operating losses. Deferred tax assets in respect of capital losses, trading losses and non-trade deficits have not been recognised as their future recovery is uncertain or not currently anticipated. The deferred tax assets not recognised are as follows: 2018 2017 £m £m Capital losses 510 362 Non-trade deficits 4 4 Trading losses 4 9 The capital losses arise in the UK and are available to carry forward indefinitely. However, the capital losses can only be offset against future capital gains. The UK non-trade deficits arose prior to 1 April 2017 and therefore can only be offset against future non-trade profits. At 31 March 2018 there was no recognised deferred tax liability for taxes that would be payable on the unremitted earnings of the Group’s subsidiaries or its associates as there are no significant corporation tax consequences of the Group’s UK, US or overseas subsidiaries or associates paying dividends to their parent companies. There are also no significant income tax consequences for the Group attaching to the payment of dividends by the Group to its shareholders. Unaudited commentary on tax Tax strategy National Grid is a responsible tax payer. Our approach to tax is consistent with the Group’s broader commitments to doing business responsibly and upholding the highest ethical standards. This includes managing our tax affairs, as we recognise that our tax contribution supports public services and the wider economy. We endeavour to manage our tax affairs so that we pay and collect the right amount of tax, at the right time, in accordance with the tax laws in all the territories in which we operate. We will claim valid tax reliefs and incentives where these are applicable to our business operations, but only where they are widely accepted through the relevant tax legislation such as those established by government to promote investment, employment and economic growth. We have a strong governance framework and our internal control and risk management framework helps us manage risks, including tax risk, appropriately. We take a conservative approach to tax risk. However, there is no prescriptive level or pre-defined limit to the amount of acceptable tax risk. We act with openness and honesty when engaging with relevant tax authorities and seek to work with tax authorities on a real time basis. We engage proactively in developments on external tax policy and engage with relevant bodies where appropriate. Ultimate responsibility and oversight of our tax strategy and governance rests with the Finance Committee, with executive management delegated to our Finance Director. For more detailed information, please refer to our published global tax strategy on our website. Total UK tax contribution (continuing and discontinued operations combined) This year we have again disclosed additional information in respect of our total UK tax contribution for consistency and to aid transparency in an area in which there remains significant public interest. As was the case in prior years, the total amount of taxes we pay and collect in the UK year-on-year is significantly more than just the corporation tax which we pay on our UK profits. Within the total, we again include other taxes paid such as business rates and taxes on employment together with employee taxes and other indirect taxes. The most significant amounts making up the 2017 /18 total tax contribution are shown in the charts below: For 2017 /18 our total tax contribution to the UK Exchequer was £0.9 billion ( 2016 /17: £1.5 billion ), taxes borne in 2017 /18 were £336 million ( 2016 /17: £644 million ) and taxes collected were £603 million ( 2016 /17: £887 million ). The fall in our total tax contribution against prior year is primarily due to the sale of the UK Gas Distribution business in the previous year, which reduced the overall size of the UK business. This reduction was expected and as such forecasted in the total UK tax contribution commentary included in our prior year accounts. Our 2016 /17 total tax contribution of £1.5 billion meant that National Grid was the 15th highest contributor of UK taxes (2015/16: 15th) based on the results of the 100 Group’s 2017 Total Tax Contribution Survey (being the most recent available), a position commensurate with the size of our business and capitalisation during that year relative to other contributors to the survey. In 2016/17 we ranked 9th in respect of taxes borne (2015/16: 9th). Due to the sale of the UK Gas Distribution business on 31 March 2017, our ranking in future 100 Group surveys is expected to fall, commensurate with the ongoing size of the business. National Grid’s contribution to the UK economy is again broader than just the taxes it pays over to and collects on behalf of HMRC. The 100 Group’s 2017 Total Tax Contribution Survey ranks National Grid in 3rd place in respect of UK capital expenditure on fixed assets, retaining 3rd place from 2016. National Grid’s economic contribution also supports a significant number of UK jobs in our supply chain. UK total tax contribution 2017 /18 (taxes paid/collected) Taxes borne £m 1. VAT 2 2. PAYE and NIC 52 3. UK corporation tax 37 4. Business rates 222 5. Other 23 Total 336 Taxes collected £m 1. VAT 478 2. PAYE and NIC 125 Total 603 Unaudited commentary on tax continued Tax transparency The UK tax charge for the year disclosed in the financial statements in accordance with accounting standards and the UK corporation tax paid during the year will differ. To aid transparency we have included a reconciliation below of the tax charge per the income statement to the UK corporation tax paid in 2017/18. The tax credit for the Group from continuing operations as reported in the income statement is £884 million ( 2016 /17: £374 million charge). The UK tax charge is £250 million ( 2016 /17: £151 million ) and UK corporation tax paid was £37 million ( 2016 /17: £129 million ), with the principal differences between these two measures as follows: Year ended 31 March 2018 2017 Reconciliation on continuing operations of UK total tax charge to UK corporation tax paid £m £m Total UK tax charge (current tax £187m 250 151 Adjustment for non-cash deferred tax (63 ) 27 Adjustments for current tax credit in respect of prior years 18 47 UK current tax charge 205 225 UK corporation tax instalment payments not payable until the following year (101 ) (216 ) UK corporation tax instalment (refunds)/payments in respect of prior years paid in current year (67 ) 120 UK corporation tax paid 37 129 Tax losses We have total unrecognised deferred tax assets in respect of losses of £518 million ( 2016 /17: £375 million ) of which £510 million ( 2016 /17: £362 million ) are capital losses in the UK as set out on page 120. These losses arose as a result of the disposal of certain businesses or assets and may be available to offset against future capital gains in the UK. Development of future tax policy We believe that the continued development of a coherent and transparent tax policy in the UK is critical to help drive growth in the economy. We continue to contribute to research into the structure of business tax and its economic impact by contributing to the funding of the Oxford University Centre for Business Tax at the Saïd Business School. We are a member of a number of industry groups which participate in the development of future tax policy, including the 100 Group, which represents the views of Finance Directors of FTSE 100 companies and several other large UK companies. Our Finance Director is Chairman of the 100 Group. This helps to ensure that we are engaged at the earliest opportunity on tax issues which affect our business. In the current year we have reviewed and responded to a number of HMRC consultations, the subject matter of which directly impacts taxes borne or collected by our business, with the aim of openly contributing to the debate and development of UK tax legislation. We undertake similar activities in the US, where the Company is an active member in the Edison Electric Institute, the American Gas Association and the Organization for International Investment.

Earnings per share (EPS)

Earnings per share (EPS)	12 Months Ended
	Mar. 31, 2018
Earnings per share [abstract]
Earnings per share (EPS)	Earnings per share (EPS) EPS is the amount of post-tax profit attributable to each ordinary share. Basic EPS is calculated on profit for the year attributable to equity shareholders divided by the weighted average number of shares in issue during the year. Diluted EPS shows what the impact would be if all outstanding share options were exercised and treated as ordinary shares at year end. The weighted average number of shares is increased by additional shares issued as scrip dividends and reduced by shares repurchased by the Company during the year. The earnings per share calculations are based on profit after tax attributable to equity shareholders of the Company which excludes non-controlling interests. Adjusted earnings and EPS, which exclude exceptional items and remeasurements, are provided to reflect the business performance sub-totals used by the Company. We have included reconciliations from this additional EPS measure to earnings for both basic and diluted EPS to provide additional detail for these items. For further details of exceptional items and remeasurements, see note 4. Following the sale of the UK Gas Distribution business on 31 March 2017, National Grid plc returned approximately £3,170 million of proceeds to shareholders through a special dividend, paid on 2 June 2017. In order to maintain the comparability of the Company’s share price before and after the special dividend, this was preceded by a share consolidation undertaken on 22 May 2017, replacing every 12 existing ordinary shares with 11 new ordinary shares. The weighted average number of ordinary shares outstanding for the period includes the effect of both the share consolidation and the special dividend from the date that the special dividend was paid. The associated share buyback programme which commenced on 2 June 2017 is now complete. Purchased shares are held as treasury shares. (a) Basic earnings per share Earnings Earnings per share Earnings Earnings per share Earnings Earnings per share 2018 2018 2017 2017 2016 2016 £m pence £m pence £m pence Adjusted earnings from continuing operations 2,060 59.5 2,141 56.9 1,812 48.0 Exceptional items after tax from continuing operations 1,319 38.1 (312 ) (8.3 ) 162 4.3 Remeasurements after tax from continuing operations 212 6.2 (19 ) (0.5 ) (73 ) (1.9 ) Earnings from continuing operations 3,591 103.8 1,810 48.1 1,901 50.4 Adjusted earnings from discontinued operations — — 607 16.1 574 15.2 Exceptional items and remeasurements after tax from discontinued operations (41 ) (1.2 ) 5,378 142.9 116 3.1 Earnings from discontinued operations (41 ) (1.2 ) 5,985 159.0 690 18.3 Total adjusted earnings 2,060 59.5 2,748 73.0 2,386 63.2 Total exceptional items and remeasurements after tax (including discontinued operations) 1,490 43.1 5,047 134.1 205 5.5 Total earnings 3,550 102.6 7,795 207.1 2,591 68.7 2018 2017 2016 millions millions millions Weighted average number of ordinary shares – basic 3,461 3,763 3,774 (b) Diluted earnings per share Earnings Earnings per share Earnings Earnings per share Earnings Earnings per share 2018 2018 2017 2017 2016 2016 £m pence £m pence £m pence Adjusted earnings from continuing operations 2,060 59.3 2,141 56.7 1,812 47.8 Exceptional items after tax from continuing operations 1,319 37.9 (312 ) (8.3 ) 162 4.3 Remeasurements after tax from continuing operations 212 6.1 (19 ) (0.5 ) (73 ) (1.9 ) Earnings from continuing operations 3,591 103.3 1,810 47.9 1,901 50.2 Adjusted earnings from discontinued operations — — 607 16.0 574 15.1 Exceptional items and remeasurements after tax from discontinued operations (41 ) (1.2 ) 5,378 142.3 116 3.1 Earnings from discontinued operations (41 ) (1.2 ) 5,985 158.3 690 18.2 Total adjusted earnings 2,060 59.3 2,748 72.7 2,386 62.9 Total exceptional items and remeasurements after tax (including discontinued operations) 1,490 42.8 5,047 133.5 205 5.5 Total earnings 3,550 102.1 7,795 206.2 2,591 68.4 2018 2017 2016 millions millions millions Weighted average number of ordinary shares – diluted 3,476 3,780 3,790 (c) Reconciliation of basic to diluted average number of shares 2018 2017 2016 millions millions millions Weighted average number of ordinary shares – basic 3,461 3,763 3,774 Effect of dilutive potential ordinary shares – employee share plans 15 17 16 Weighted average number of ordinary shares – diluted 3,476 3,780 3,790

Dividends

Dividends	12 Months Ended
	Mar. 31, 2018
Subclassifications of assets, liabilities and equities [abstract]
Dividends	Dividends Dividends represent the return of profits to shareholders. Dividends are paid as an amount per ordinary share held. We retain part of the profits generated in the year to meet future growth plans and pay out the remainder in accordance with our dividend policy. Interim dividends are recognised when they become payable to the Company’s shareholders. Final dividends are recognised when they are approved by shareholders. 2018 2017 2016 Pence per share Cash dividend paid £m Scrip dividend £m Pence per share Cash dividend paid £m Scrip dividend £m Pence per share Cash dividend paid £m Scrip dividend £m Interim dividend in respect of the current year 15.49 346 176 15.17 540 32 15.00 532 31 Special dividend 84.375 3,171 — — — — — — — Final dividend in respect of the prior year 29.10 970 33 28.34 923 151 28.16 805 248 128.965 4,487 209 43.51 1,463 183 43.16 1,337 279 The Directors are proposing a final dividend for the year ended 31 March 2018 of 30.44 p per share that will absorb approximately £1.0 billion of shareholders’ equity (assuming all amounts are settled in cash). It will be paid on 15 August 2018 to shareholders who are on the register of members at 1 June 2018 (subject to shareholders’ approval at the AGM). A scrip dividend will be offered as an alternative. Following completion of the sale of the majority interest in UK Gas Distribution, the Company paid a special dividend on 2 June 2017 of 84.375 p per existing ordinary share ( $5.4224 per existing American Depositary Share). This returned approximately £3,170 million to shareholders. No scrip dividend was offered as an alternative. Unaudited commentary on dividends Following the announcement of our dividend policy in March 2013, the Board remains confident that National Grid is able to support a dividend per share growing at least in line with RPI inflation for the foreseeable future, while continuing to invest as required in our regulated assets. In August 2014 we began a share buyback programme that will allow us to offer the scrip dividend option for both the full year and interim dividend. The buyback programme is designed to balance shareholders’ appetite for the scrip dividend option with our desire to operate an efficient balance sheet with appropriate leverage. At this stage we do not expect to buyback scrip issuances in 2019 and 2020, unless we have higher than anticipated balance sheet capacity.

Discontinued operations

Discontinued operations	12 Months Ended
	Mar. 31, 2018
Non-current Assets Held For Sale And Discontinued Operations [Abstract]
Discontinued operations	Discontinued operations Our results and cash flows of significant assets or businesses sold during the year are shown separately from our continuing operations. Assets and businesses are classified as held for sale when their carrying amounts are recovered through sale rather than through continuing use. It only meets the held for sale condition when the assets are ready for immediate sale in their present condition, management is committed to the sale and it is highly probable that the sale will complete within one year. Depreciation ceases on assets and businesses when they are classified as held for sale and the assets and businesses are impaired if the proceeds less sale costs fall short of the carrying value. As a result of the sale of a 61% controlling interest in UK Gas Distribution on 31 March 2017, we are required to report our earnings for the Group excluding UK Gas Distribution (‘continuing operations’) separately from the results of that business, which we report within ‘discontinued operations’. The gain recognised by the Group on sale is analysed in the detail of the note below. All costs associated with the transaction, including those associated with separation and setting up UK Gas Distribution are shown as a deduction from the proceeds received. Any adjustments arising as part of the completion adjustments finalised within the measurement period would result in a further gain or loss on disposal to be reported within discontinued operations in the current period. Disposal of UK Gas Distribution On 31 March 2017 the Group completed the disposal of a 61% equity interest in the UK Gas Distribution business, principally comprising the Group’s equity and debt interests in National Grid Gas Distribution Limited together with certain other assets (principally property and a 45% interest in Xoserve Limited). Further details are included in the Annual Report and Accounts 2016/17. The Group sold its 100% equity interest in UK Gas Distribution to Quadgas HoldCo Limited, a newly incorporated UK limited company 61% owned by Quadgas Investments BidCo Limited and 39% by the Group’s subsidiary National Grid Holdings One plc. In exchange, the Group received cash consideration of £3,679 million , loan proceeds of £1,775 million and recognised a shareholder loan receivable of £429 million and a 39% equity interest in Quadgas HoldCo Limited. The UK Gas Distribution business met the criteria to be classified as held for sale at 8 December 2016, the date that the Group initially entered into the sale agreement, and depreciation and amortisation (circa £25 million per month) on tangible and intangible fixed assets ceased from this date. The disposal of UK Gas Distribution resulted in a £5.3 billion gain on disposal. The provisional purchase price allocation reported in the Annual Report and Accounts 2016/17 has been finalised and there were no significant adjustments arising on finalisation of this exercise in the current year. The business represented a reportable segment and a separate major line of business and accordingly was presented as a discontinued operation in the consolidated income statement, consolidated statement of comprehensive income and the consolidated cash flow statement in 2016/17. In 2017/18 a loss of £41 million is reported in discontinued operations, with £33 million relating to the completion accounts settlement in November 2017. In addition, this reflects a net charge of £8 million representing further transaction costs and gains principally relating to the reversal of provisions. In addition, there was a cash outflow from operating activities of £207 million related to the utilisation of provisions, principally relating to payments of professional fees in respect of the disposal of the UK Gas Distribution business. Net cash flows used in financing activities were £231 million for the settlement of RPI swaps relating to the final stages of the Group-wide liability management programme executed as part of the sale process ( 2017 : cash flows comprising £4.8 billion of debt issued and term debt raised, offset by £3.2 billion in respect of bond buybacks). On 1 May 2018, the Group announced that it had entered into an agreement with Quadgas Investments BidCo Limited regarding the potential sale of its remaining 25% interest in Quadgas HoldCo Limited. Further details are given in notes 4, 15 and 35. 9. Discontinued operations continued Summary income statement – discontinued operations The summary income statement for discontinued operations for the years ended 31 March 2017 and 2016 are as follows: 2017 2016 £m £m Revenue 1,887 1,903 Operating costs (993 ) (1,043 ) Operating profit 1 894 860 Finance costs (152 ) (157 ) Profit before tax from discontinued operations 742 703 Tax from discontinued operations (79 ) (11 ) Profit after tax from discontinued operations 663 692 Gain on disposal of UK Gas Distribution 5,009 — Tax on gain on disposal of UK Gas Distribution 312 — Gain on disposal of UK Gas Distribution after tax 5,321 — Total profit after tax from discontinued operations 5,984 692 1. 2016 includes sale preparation costs of £22 million in respect of the disposal of the UK Gas Distribution business. 2017 costs have been included as part of transaction costs in determining the gain on disposal. The total gain on the disposal after tax of £5,321 million was comprised of total consideration of £7,494 million before transaction costs of £1,837 million and a tax credit of £312 million compared to net assets on disposal of £648 million . Statement of comprehensive income – discontinued operations for the years ended 31 March 2017 2016 Notes £m £m Profit after tax from discontinued operations 5,984 692 Other comprehensive (loss)/income Items that will never be reclassified to profit or loss: Remeasurement (losses)/gains of pension assets and post-retirement benefit obligations 23 (75 ) 129 Tax on items that will never be reclassified to profit or loss 6 13 (30 ) Total items from discontinued operations that will never be reclassified to profit or loss (62 ) 99 Items that may be reclassified subsequently to profit or loss: Net losses in respect of cash flow hedges (106 ) (38 ) Transferred to profit or loss in respect of cash flow hedges 233 3 Tax on items that may be reclassified subsequently to profit or loss 6 (23 ) 7 Total items from discontinued operations that may be reclassified subsequently to profit or loss 104 (28 ) Other comprehensive income/(loss) for the year, net of tax from discontinued operations 42 71 Total comprehensive income for the year from discontinued operations 6,026 763

Goodwill

Goodwill	12 Months Ended
	Mar. 31, 2018
Intangible Assets [Abstract]
Goodwill	Goodwill Goodwill represents the excess of what we paid to acquire businesses over the fair value of their net assets at the acquisition date. We assess whether goodwill is recoverable each year by performing an impairment review. Goodwill is recognised as an asset and is not amortised, but is tested for impairment annually or more frequently if events or changes in circumstances indicate a potential impairment. Any impairment is recognised immediately in the income statement and is not subsequently reversed. Goodwill and fair value adjustments arising on the acquisition of a foreign entity are treated as assets and liabilities of the foreign entity and translated at the closing exchange rate. Goodwill is allocated to cash-generating units and this allocation is made to those cash-generating units that are expected to benefit from the business combination in which the goodwill arose. Impairment is recognised where there is a difference between the carrying value of the cash-generating unit and the estimated recoverable amount of the cash-generating unit to which that goodwill has been allocated. Any impairment loss is first allocated to the carrying value of the goodwill and then to the other assets within the cash-generating unit. Recoverable amount is defined as the higher of fair value less costs to sell and estimated value-in-use at the date the impairment review is undertaken. Value-in-use represents the present value of expected future cash flows, discounted using a pre-tax discount rate that reflects current market assessments of the time value of money and the risks specific to the asset for which the estimates of future cash flows have not been adjusted. Impairments are recognised in the income statement and are disclosed separately. Total £m Net book value at 1 April 2016 5,315 Exchange adjustments 781 Net book value at 31 March 2017 6,096 Exchange adjustments (652 ) Net book value at 31 March 2018 5,444 The cost of goodwill at 31 March 2018 was £5,458 million (2017: £6,112 million ) with an accumulated impairment charge of £14 million (2017: £16 million ). The amounts disclosed above as at 31 March 2018 include balances relating to the following cash-generating units: New York £3,137 million (2017: £3,512 million ); Massachusetts £1,173 million (2017: £1,313 million ); Rhode Island £436 million (2017: £488 million ); and Federal £699 million (2017: £783 million ). Goodwill is reviewed annually for impairment and the recoverability of goodwill has been assessed by comparing the carrying amount of our operations described above (our cash-generating units) with the expected recoverable amount on a value-in-use basis. In each assessment, the value-in-use has been calculated based on five-year plan projections that incorporate our best estimates of future cash flows, customer rates, costs (including changes in commodity prices), future prices and growth. Such projections reflect our current regulatory rate plans taking into account regulatory arrangements to allow for future rate plan filings and recovery of investment. Our plans have proved to be reliable guides in the past and the Directors believe the estimates are appropriate. The future economic growth rate used to extrapolate projections beyond five years is 2.3% (2017: 2.0% ). The growth rate has been determined having regard to data on projected growth in US real gross domestic product (GDP). Based on our business’ place in the underlying US economy, it is appropriate for the terminal growth rate to be based upon the overall growth in real GDP and, given the nature of our operations, to extend over a long period of time. Cash flow projections have been discounted to reflect the time value of money, using a post-tax discount rate of 5.3% (2017: 5.4% ). The equivalent pre-tax discount rate is 5.3% (2017: 5.4% ) as no cash tax is payable in our five-year plan projections. The discount rate represents the estimated weighted average cost of capital of these operations. In reaching this conclusion, the Directors have considered the potential future consequences regarding the manner in which Tax Reform will impact the Group and its future cash flows. The full implications of the new legislation on earnings and cash flows are still being reviewed, and will depend on the outcome of discussions with regulators. In our US business, we are subject to federal and state taxes, however our regulatory arrangements require us to pass this cost back to our customers. The reduction in the corporation tax rate from 35% to 21% will be reflected through lower bills to customers, reducing our revenues (and tax costs) in future periods. For the purposes of the goodwill impairment exercise, we have reflected the lower billing levels through lower revenue forecasts as well as lower tax charges. Historically, as a result of tax losses arising from claiming accelerated depreciation allowances, we have not paid substantial amounts of tax in the US. Accordingly, for IFRS purposes, we have recognised significant deferred tax liabilities in respect of these accelerated allowances. In accounting terms, Tax Reform triggers the remeasurement of our deferred tax liabilities from 35% to 21% which has resulted in the exceptional gain under IFRS (as disclosed in notes 4 and 6). However, the impact for our US business is that the amounts we have previously received from customers assuming a 35% federal tax rate must now be returned to customers. The precise manner and timing over which this occurs remains subject to agreement with our regulators. We are currently in discussions with regulators as to how best to adjust the customer bills for this purpose, and over what time period. Offsetting this change will be the additional income we earn, since the rate base will grow faster. (Our rate base is net of deferred tax liabilities, which, as a result of Tax Reform, will now be smaller.) In overall terms we expect the outcome to be economically neutral. In assessing the carrying value of goodwill, we have sensitised our forecasts to factor in a reduction in revenues and lower tax costs into our cash flow forecasts, but not reflected the impact of additional rate base growth on future earnings. While it is possible that a key assumption in the calculation could change, the Directors believe that no reasonably foreseeable change would result in an impairment of goodwill, in view of the long-term nature of the key assumptions and the margin by which the estimated value-in-use exceeds the carrying amount.

Other intangible assets

Other intangible assets	12 Months Ended
	Mar. 31, 2018
Intangible Assets [Abstract]
Other intangible assets	Other intangible assets Other intangible assets include software which is written down (amortised) over the length of period we expect to receive a benefit from the asset. Identifiable intangible assets are recorded at cost less accumulated amortisation and any provision for impairment. Other intangible assets are tested for impairment only if there is an indication that the carrying value of the assets may have been impaired. Impairments of assets are calculated as the difference between the carrying value of the asset and the recoverable amount, if lower. Where such an asset does not generate cash flows that are independent from other assets, the recoverable amount of the cash-generating unit to which that asset belongs is estimated. Impairments are recognised in the income statement and are disclosed separately. Any assets which suffered impairment in a previous period are reviewed for possible reversal of the impairment at each reporting date. Internally generated intangible assets, such as software, are recognised only if: an asset is created that can be identified; it is probable that the asset created will generate future economic benefits; and the development cost of the asset can be measured reliably. Where no internally generated intangible asset can be recognised, development expenditure is recorded as an expense in the period in which it is incurred. Other intangible assets are amortised on a straight-line basis over their estimated useful economic lives. Amortisation periods for categories of intangible assets are: Years Software 3 to 10 Software £m Cost at 1 April 2016 1,744 Exchange adjustments 105 Additions 234 Disposals (43 ) Disposal of UK Gas Distribution (304 ) Reclassifications 1 (4 ) Cost at 31 March 2017 1,732 Exchange adjustments (98 ) Additions 173 Disposals (18 ) Reclassifications 1 8 Cost at 31 March 2018 1,797 Accumulated amortisation at 1 April 2016 (857 ) Exchange adjustments (43 ) Amortisation charge for the year (164 ) Accumulated amortisation of disposals 40 Disposal of UK Gas Distribution 215 Accumulated amortisation at 31 March 2017 (809 ) Exchange adjustments 43 Amortisation charge for the year (138 ) Accumulated amortisation of disposals 6 Accumulated amortisation at 31 March 2018 (898 ) Net book value at 31 March 2018² 899 Net book value at 31 March 2017 923 1. Reclassifications includes amounts transferred (to)/from property, plant and equipment (see note 12). 2. Included in software is £160 million relating to the US ERP system, which still has a remaining amortisation period of six years.

Property, plant and equipment

Property, plant and equipment	12 Months Ended
	Mar. 31, 2018
Property, plant and equipment [abstract]
Property, plant and equipment	Property, plant and equipment The following note shows the physical assets controlled by us. The cost of these assets primarily represents the amount initially paid for them. This includes both their purchase price and the construction and other costs associated with getting them ready for operation. A depreciation expense is charged to the income statement to reflect annual wear and tear and the reduced value of the asset over time. Depreciation is calculated by estimating the number of years we expect the asset to be used (useful economic life) and charging the cost of the asset to the income statement equally over this period. Our strategy in action We operate an energy networks business and therefore have a significant physical asset base. We continue to invest in our networks to maintain reliability, create new customer connections and ensure our networks are flexible and resilient. Our business plan envisages these additional investments will be funded through a mixture of cash generated from operations and the issue of new debt. Property, plant and equipment is recorded at cost, less accumulated depreciation and any impairment losses. Cost includes the purchase price of the asset, any payroll and finance costs incurred which are directly attributable to the construction of property, plant and equipment as well as the cost of any associated asset retirement obligations. Property, plant and equipment includes assets in which the Group’s interest comprises legally protected statutory or contractual rights of use. Additions represent the purchase or construction of new assets, including capital expenditure for safety and environmental assets, and extensions to, enhancements to, or replacement of existing assets. All costs associated with projects or activities which have not been fully commissioned at the period end are classified within assets in the course of construction. Contributions received prior to 1 July 2009 towards the cost of property, plant and equipment are included in trade and other payables as deferred income and credited on a straight-line basis to the income statement over the estimated useful economic lives of the assets to which they relate. Contributions received post 1 July 2009 are recognised in revenue immediately, except where the contributions are consideration for a future service, in which case they are recognised initially as deferred income and revenue is subsequently recognised over the period in which the service is provided. No depreciation is provided on freehold land or assets in the course of construction. Other items of property, plant and equipment are depreciated, on a straight-line basis, at rates estimated to write off their book values over their estimated useful economic lives. In assessing estimated useful economic lives, consideration is given to any contractual arrangements and operational requirements relating to particular assets. The assessments of estimated useful economic lives and residual values of assets are performed annually. Unless otherwise determined by operational requirements, the depreciation periods for the principal categories of property, plant and equipment are, in general, as shown in the table below: Years Freehold and leasehold buildings up to 101 Plant and machinery: Electricity transmission plant and wires 15 to 10 0 Electricity distribution plant 29 to 75 Electricity generation plant 20 to 93 Interconnector plant and other 5 to 60 Gas plant – mains, services and regulating equipment 10 to 95 Gas plant – storage 5 to 65 Gas plant – meters 7 to 65 Motor vehicles and office equipment up to 29 Gains and losses on disposals are determined by comparing the proceeds with the carrying amount and are recognised within operating profit in the income statement. Items within property, plant and equipment are tested for impairment only if there is some indication that the carrying value of the assets may have been impaired. Impairments of assets are calculated as the difference between the carrying value of the asset and the recoverable amount, if lower. Where such an asset does not generate cash flows that are independent from other assets, the recoverable amount of the cash-generating unit to which that asset belongs is estimated. Impairments are recognised in the income statement and if immaterial are included within depreciation charge for the year. Any assets which suffered impairment in a previous period are reviewed for possible reversal of the impairment at each reporting date. 12. Property, plant and equipment continued Land and buildings £m Plant and machinery £m Assets in the course of construction £m Motor vehicles and office equipment £m Total £m Cost at 1 April 2016 2,758 54,772 3,874 1,171 62,575 Exchange adjustments 196 3,157 93 76 3,522 Additions 55 822 3,080 132 4,089 Disposals 1 (22 ) (572 ) (70 ) (204 ) (868 ) Disposal of UK Gas Distribution (112 ) (11,861 ) (88 ) (300 ) (12,361 ) Reclassifications 2 104 2,913 (2,938 ) (41 ) 38 Cost at 31 March 2017 2,979 49,231 3,951 834 56,995 Exchange adjustments (169 ) (2,862 ) (89 ) (67 ) (3,187 ) Additions 38 430 3,358 75 3,901 Disposals 1 (16 ) (216 ) (21 ) (34 ) (287 ) Reclassifications 2 98 2,791 (2,926 ) 49 12 Cost at 31 March 2018 2,930 49,374 4,273 857 57,434 Accumulated depreciation at 1 April 2016 (640 ) (17,828 ) — (743 ) (19,211 ) Exchange adjustments (29 ) (780 ) — (44 ) (853 ) Depreciation charge for the year (84 ) (1,338 ) — (113 ) (1,535 ) Disposals 1 42 545 — 203 790 Disposal of UK Gas Distribution 29 3,425 — 207 3,661 Reclassifications 2 (2 ) (20 ) — — (22 ) Accumulated depreciation at 31 March 2017 (684 ) (15,996 ) — (490 ) (17,170 ) Exchange adjustments 28 695 — 36 759 Depreciation charge for the year (28 ) (1,276 ) — (88 ) (1,392 ) Disposals 1 10 199 — 33 242 Reclassifications 2 — (20 ) — — (20 ) Accumulated depreciation at 31 March 2018 (674 ) (16,398 ) — (509 ) (17,581 ) Net book value at 31 March 2018 2,256 32,976 4,273 348 39,853 Net book value at 31 March 2017 2,295 33,235 3,951 344 39,825 1. Includes the reversal of assets with cost of £51 million ( 2017 : £107 million ) and accumulated depreciation of £51 million ( 2017 : £107 million ) disposed in previous years that remain in use in the Group. It also includes £334 million of adjustments from accumulated depreciation to cost for historical disposals relating to assets acquired as part of the KeySpan acquisition in 2008 which were disposed of in subsequent periods. Both of these adjustments have a nil net book value impact. 2. Represents amounts transferred between categories, (to)/from other intangible assets (see note 11), reclassifications from inventories and reclassifications between cost and accumulated depreciation. 2018 2017 £m £m Information in relation to property, plant and equipment Capitalised interest included within cost 1,861 1,749 Net book value of assets held under finance leases (all relating to motor vehicles and office equipment) 253 289 Additions to assets held under finance leases (all relating to motor vehicles and office equipment) 58 98 Contributions to cost of property, plant and equipment included within: Trade and other payables 85 89 Non-current liabilities 844 839

Other non-current assets

Other non-current assets	12 Months Ended
	Mar. 31, 2018
Subclassifications of assets, liabilities and equities [abstract]
Other non-current assets	Other non-current assets Other non-current assets include assets that do not fall into any other non-current asset category (such as goodwill or property, plant and equipment) where the benefit to be received from the asset is not due to be received until after 31 March 2019. 2018 2017¹ £m £m Other receivables 36 45 Non-current tax assets 51 — Prepayments and accrued income 28 24 115 69 1. Comparative amounts have been represented to reflect the reclassification of commodity derivative contracts from other non-current assets to derivatives (see note 16).

Financial and other investments

Financial and other investments	12 Months Ended
	Mar. 31, 2018
Disclosure of financial assets [abstract]
Financial and other investments	Financial and other investments Financial and other investments include three main categories. Assets classified as available-for-sale typically represent investments in short-term money funds and quoted investments in equities or bonds of other companies. The second category comprises long-term loans to our associates and joint ventures. The third category is other loans and receivables which includes bank deposits with a maturity of greater than three months, and cash balances that cannot be readily used in operations, principally collateral pledged against derivative holdings. Financial assets, liabilities and equity instruments are classified according to the substance of the contractual arrangements entered into, and recognised on trade date. Available-for-sale financial assets are non-derivatives that are either designated in this category or not classified in any other categories. Available-for-sale financial investments are recognised at fair value plus directly related incremental transaction costs, and are subsequently carried at fair value in the statement of financial position. Changes in the fair value of available-for-sale investments are recognised directly in other comprehensive income, until the investment is disposed of or is determined to be impaired. At this time the cumulative gain or loss previously recognised in equity is included in the income statement for the period. Investment income is recognised using the effective interest method and taken through interest income in the income statement. Loans receivable and other receivables are initially recognised at fair value plus transaction costs and subsequently held at amortised cost using the effective interest method. Interest income, together with gains and losses when the loans and receivables are derecognised or impaired, are recognised in the income statement. Subsequent to initial recognition, the fair values of financial assets measured at fair value that are quoted in active markets are based on bid prices. When independent prices are not available, fair values are determined by using valuation techniques that are consistent with techniques commonly used by the relevant market. The techniques use observable market data. 2018 2017 £m £m Non-current Available-for-sale investments 417 605 Loans to joint ventures and associates 1 482 495 899 1,100 Current Available-for-sale investments 2,304 7,432 Other loans and receivables 390 1,309 2,694 8,741 3,593 9,841 Financial and other investments include the following: Investments in short-term money funds 2 1,999 6,899 Managed investments in equity and bonds 3 530 939 Cash surrender value of life insurance policies 198 202 Loans to joint ventures and associates 482 495 Restricted balances: Collateral 4 335 1,262 Other 49 44 3,593 9,841 1. Comprises £352 million ( 2017 : £434 million ) relating to a shareholder loan to Quadgas HoldCo Limited, and the remainder is a loan to a joint venture. 2. Includes £69 million ( 2017 : £14 million ) held by insurance captives and therefore restricted. 3. Includes restricted amounts of £301 million ( 2017 : £434 million ) held by insurance captives and £214 million ( 2017 : £225 million ) relating to US non-qualified plan investments. 4. Refers to collateral placed with counterparties with whom we have entered into a credit support annex to the ISDA (International Swaps and Derivatives Association) Master Agreement. Available-for-sale investments are recorded at fair value. The carrying value of current loans and receivables is approximate to their fair values, due to short-dated maturities. The carrying value of the non-current loans to joint ventures and associates approximates their fair values as at 31 March 2018 and 31 March 2017. The exposure to credit risk at the reporting date is the fair value of the financial investments. For further information on our credit risk, refer to note 30(a). None of the financial investments are impaired. Unaudited commentary on financial and other investments Current available-for-sale investments at 31 March 2018 were £5,128 million lower than the prior year. The balance at 31 March 2017 included the proceeds received from the sale of the UK Gas Distribution business. £4,010 million of these proceeds were distributed in the current year through the special dividend and share buyback.

Investments in joint ventures a

Investments in joint ventures and associates	12 Months Ended
	Mar. 31, 2018
Interests In Other Entities [Abstract]
Investments in joint ventures and associates	Investments in joint ventures and associates Investments in joint ventures and associates represent businesses we do not control, but instead exercise joint control or significant influence. A joint venture is an arrangement established to engage in economic activity, which the Group jointly controls with other parties and has rights to the net assets of the arrangement. An associate is an entity which is neither a subsidiary nor a joint venture, but over which the Group has significant influence. 2018 2017 Associates £m Joint ventures £m Total £m Associates £m Joint ventures £m Total £m Share of net assets at 1 April 1,776 307 2,083 84 313 397 Exchange adjustments (19 ) 7 (12 ) 16 19 35 Additions 65 64 129 74 63 137 Acquisition of stake in Quadgas HoldCo Limited — — — 1,611 — 1,611 Capitalisation of shareholder loan to Quadgas HoldCo Limited 69 — 69 — — — Impairment charge against investment in Quadgas HoldCo Limited (213 ) — (213 ) — — — Share of post-tax results for the year 147 26 173 15 48 63 Share of other comprehensive income of associates, net of tax 147 — 147 — — — Dividends received (170 ) (43 ) (213 ) (24 ) (75 ) (99 ) Other movements 5 — 5 — (61 ) (61 ) Share of net assets at 31 March 1,807 361 2,168 1,776 307 2,083 A list of joint ventures and associates including the name and proportion of ownership is provided in note 32. Further information on the Group’s acquisition of an associate interest in Quadgas HoldCo Limited is provided in note 9 and opposite. On 1 May 2018, the Group announced that it had entered into the Remaining Acquisition Agreement (RAA) with Quadgas Investments BidCo Limited (the Consortium) regarding the potential sale of its remaining 25% interest in Quadgas HoldCo Limited, the holding company for Cadent Gas Limited. The RAA contains both put and call options for both the Group and the Consortium that can be exercised in the period between 1 March 2019 and 31 October 2019 (subject to giving six months’ notice) for cash proceeds of approximately £1.2 billion . The nature of the contractual arrangements in the RAA are substantially the same as those within the Further Acquisition Agreement (FAA), which also contains put and call options that can be exercised by either party during the same time-frame. The carrying value of the Group’s equity interest in Quadgas HoldCo Limited at 31 March 2017 was £1.6 billion , reflecting our best estimate of the fair value of that stake, as it was treated as an asset acquired at fair value following the disposal of a controlling interest in the UK Gas Distribution business. As set out in note 16, at 31 March 2018 we recognised a derivative financial asset of £110 million relating to the FAA agreement. In addition, an impairment review of the Group’s interests in Quadgas HoldCo Limited, (comprising the FAA derivative, a 39% equity interest and £352 million in shareholder loans), was undertaken, comparing the aggregated carrying value of these interests against the future dividend income and proceeds we would expect to receive under the FAA and RAA. This resulted in a charge of £213 million , recorded as impairment against the carrying value of the equity. The impairment largely offsets pension accounting gains, and the recognition of the FAA derivative asset. Following recognition of the FAA derivative asset and the impairment charge, the aggregate carrying value of Group’s interests associated with Quadgas HoldCo Limited is £2.1 billion , which is close to our original assessment of fair market value as determined at 31 March 2017. Of this, £1.6 billion relates to our equity interest. In 2017 the Group first entered into an arrangement with San Francisco-based Sunrun Neptune Investor 2016 LLC, a leading US provider of residential solar energy systems to provide investment capital. In the period to 31 March 2018 , the Group invested £38 million ( 2017 : £41 million ) alongside Sunrun into a newly incorporated partnership vehicle. The investment is classified as an associate as the Group has significant influence over the activities of the partnership vehicle. The fair value gain on this investment of £7 million has been reflected within the share of post-tax results for the year. The joint ventures and associates have no significant contingent liabilities to which the Group is exposed, and the Group has no significant contingent liabilities in relation to its interests in the joint ventures and associates. The Group has capital commitments of £120 million ( 2017 : £235 million ) in relation to joint ventures. Outstanding balances with joint ventures and associates are shown in note 29. At 31 March 2018 , the Group had one material joint venture, being its 50% equity stake in BritNed Development Limited, and one material associate being its 39% equity stake in Quadgas HoldCo Limited. 15. Investments in joint ventures and associates continued BritNed Development Limited (joint venture) BritNed Development Limited is a joint venture with transmission system operator TenneT and operates the subsea electricity link between Great Britain and the Netherlands, commissioned in 2011 . BritNed Development Limited has a reporting period end of 31 December with monthly management reporting information provided to National Grid. Summarised financial information of this joint venture, as at 31 March, together with the carrying amount of the investment in the consolidated financial statements is as follows: 2018 2017 £m £m Statement of financial position – BritNed Development Limited Non-current assets 390 392 Cash and cash equivalents 50 45 All other current assets 4 1 Non-current liabilities (10 ) (10 ) Current liabilities (28 ) (20 ) Equity 406 408 Carrying amount of the Group’s investment (National Grid ownership 50%) 203 204 2018 2017 £m £m Income statement – BritNed Development Limited Revenue 429 399 Depreciation and amortisation (13 ) (13 ) Other costs (324 ) (257 ) Operating profit 92 129 Income tax expense (20 ) (23 ) Profit for the year 72 106 Group’s share of profit (National Grid ownership 50%) 36 53 Quadgas HoldCo Limited (associate) As set out in note 9, on disposal of the Group’s interest in the UK Gas Distribution business, the Group retained an equity interest in UK Gas Distribution through its parent, Quadgas HoldCo Limited, and a shareholder loan asset of £0.4 billion (see note 14). The Group has the power to appoint 4 of the 12 members of the board of Quadgas HoldCo Limited, which confers significant influence, but not joint control. In general, the key strategic, operational and financial decisions can be effected by a simple majority of votes. However, in limited circumstances, certain decisions require the consent of both parties. While these circumstances are not expected to occur regularly, given the rights conferred, and in view of the Group’s equity stake, the investment has been accounted for as an equity investment in an associate. The Group initially recognised its 39% interest in Quadgas HoldCo Limited at fair value, being the market price of the investment as at 31 March 2017 . The FAA was signed concerning a 14% interest in Quadgas HoldCo Limited, which contains put and call options for both the Group and the Consortium that can be exercised in the period between 1 March 2019 and 31 October 2019. The FAA is accounted for as a derivative financial instrument (see notes 16 and 30(f)). Quadgas HoldCo Limited is an unlisted entity, and so no quoted price exists. The fair value on initial recognition was determined with reference to the equity value of the business implicit in the sale transaction, adjusted to reflect a deduction for the estimated premium paid for control by the Consortium. In assigning value to the retained interests, the Group valued 14% of its 39% interest based on the price implied by the FAA. The deduction for control premium was applied to the residual 25% interest. 15. Investments in joint ventures and associates continued The Group has completed a purchase price allocation exercise for its interest in Quadgas HoldCo Limited. There were adjustments arising from the finalisation of the purchase price allocation. A £33 million loss was recognised relating to the completion accounts settlement, resulting in a £20 million payment being made to Quadgas BidCo Limited. The fair values of the assets and liabilities of Quadgas HoldCo Limited as at 31 March 2017 , as previously reported, are set out below, along with the reconciliation to the carrying value of the investment in the associate at that date: 2017 £m Statement of financial position – Quadgas HoldCo Limited (as previously reported) Non-current assets 15,559 Current assets 299 Non-current liabilities (10,408 ) Current liabilities (519 ) Equity 4,931 Proportion of the Group’s ownership interest in associate 1,923 Discount for non-controlling interest (312 ) Carrying amount of the Group’s interest in associate (National Grid ownership 39%) 1,611 Summarised financial information of this associate (incorporating the purchase price allocation completion adjustments), as at 31 March, together with the carrying amount of the investment in the consolidated financial statements is as follows: 2018 2017 £m £m Statement of financial position – Quadgas HoldCo Limited Non-current assets 16,735 16,047 Current assets 427 299 Non-current liabilities (11,195 ) (10,864 ) Current liabilities (534 ) (551 ) Equity 5,433 4,931 Proportion of the Group’s ownership interest in associate 2,119 1,923 Discount for non-controlling interest (arising on initial acquisition) (312 ) (312 ) Impairment charge against investment (213 ) — Carrying amount of the Group’s interest in associate (National Grid ownership 39%) 1,594 1,611 The sale of the previously owned subsidiary and subsequent acquisition of the 39% equity interest occurred (see note 9) on 31 March 2017 . All profit or loss impact for the year ended 31 March 2017 is disclosed as discontinued operations. The summarised income statement for the year ended 31 March 2018 is set out below: 2018 £m Income statement – Quadgas HoldCo Limited Revenue 1,468 Depreciation and amortisation (378 ) Other costs (423 ) Operating profit 667 Net interest payable (272 ) Income tax expense (76 ) Profit for the year 319 Group’s share of profit (National Grid ownership 39%) 124 Subsidiary undertakings, joint ventures and associates While we present consolidated results in these financial statements as if we were one company, our legal structure is such that there are a number of different operating and holding companies that contribute to the overall result. This structure has evolved through acquisitions as well as regulatory requirements to have certain activities within separate legal entities. Subsidiary undertakings A list of the Group’s subsidiaries as at 31 March 2018 is given below. The entire share capital of subsidiaries is held within the Group except where the Group’s ownership percentages are shown. These percentages give the Group’s ultimate interest and therefore allow for the situation where subsidiaries are owned by partly-owned intermediate subsidiaries. Where subsidiaries have different classes of shares, this is largely for historical reasons and the effective percentage holdings given represent both the Group’s voting rights and equity holding. Shares in National Grid (US) Holdings Limited, National Grid Holdings One plc and NGG Finance plc are held directly by National Grid plc. All other holdings in subsidiaries are owned by other subsidiaries within the Group. All subsidiaries are consolidated in the Group’s financial statements. Principal Group companies are identified in bold . These companies are incorporated and principally operate in the countries under which they are shown. Incorporated in England and Wales Registered office: 1–3 Strand, London WC2N 5EH, UK (unless stated otherwise in footnotes). Beegas Nominees Limited Birch Sites Limited Carbon Sentinel Limited Droylsden Metering Services Limited Gridcom Limited Icelink Interconnector Limited Landranch Limited Lattice Group Employee Benefit Trust Limited Lattice Group Limited Lattice Group Trustees Limited Natgrid Limited NatGrid One Limited NatgridTW1 Limited National Grid Belgium Limited National Grid Blue Power Limited National Grid Carbon Limited National Grid Commercial Holdings Limited National Grid Distributed Energy Limited National Grid Electricity Group Trustee Limited National Grid Electricity System Operator Limited National Grid Electricity Transmission plc National Grid Energy Metering Limited National Grid Four Limited National Grid Fourteen Limited National Grid Gas Holdings Limited National Grid Gas plc National Grid Grain LNG Limited National Grid Holdings Limited National Grid Holdings One plc National Grid IFA 2 Limited National Grid Interconnector Holdings Limited National Grid Interconnectors Limited National Grid International Limited National Grid Metering Limited National Grid North Sea Link Limited National Grid Offshore Limited (previously Cadent Services Limited)* National Grid Property Holdings Limited National Grid Seventeen Limited National Grid Smart Limited National Grid Ten National Grid Thirty Five Limited National Grid Thirty Four Limited (previously Cadent Gas Limited)* National Grid Thirty Six Limited (previously Cadent Finance Limited)* National Grid Twelve Limited National Grid Twenty Eight Limited National Grid Twenty-Five Limited National Grid Twenty Seven Limited National Grid Twenty Three Limited National Grid UK Limited National Grid UK Pension Services Limited National Grid (US) Holdings Limited National Grid (US) Investments 2 Limited National Grid (US) Investments 4 Limited National Grid (US) Partner 1 Limited National Grid Ventures Limited National Grid Viking Link Limited National Grid William Limited NG Nominees Limited NG Shetland Link Limited NGC Employee Shares Trustee Limited NGG Finance plc Ngrid Intellectual Property Limited NGT Telecom No. 1 Limited NGT Two Limited Port Greenwich Limited Stargas Nominees Limited Supergrid Electricity Limited Supergrid Energy Transmission Limited Supergrid Limited Thamesport Interchange Limited The National Grid Group Quest Trustee Company Limited The National Grid YouPlan Trustee Limited Transco Limited Warwick Technology Park Management Company (No 2) Limited ( 60.56% ) 1 1. Registered office: Shire Hall, PO Box 9, Warwick CV34 4RL, UK * Change of name effective from 2 May 2017 32. Subsidiary undertakings, joint ventures and associates continued Incorporated in the US Registered office: Corporation Service Company, 251 Little Falls Drive, Wilmington, DE 19808, New Castle County, USA (unless stated otherwise in footnotes). Boston Gas Company 1 British Transco Capital Inc. British Transco Finance, Inc. Broken Bridge Corp. 2 Colonial Gas Company 1 EUA Energy Investment Corporation 1 GridAmerica Holdings Inc. Grid NY LLC 3 KeySpan CI Midstream Limited KeySpan Corporation 3 KeySpan Energy Corporation 3 KeySpan Energy Services Inc. KeySpan Gas East Corporation 3 KeySpan International Corporation KeySpan MHK, Inc. KeySpan Midstream Inc. KeySpan Plumbing Solutions, Inc. 3 KSI Contracting, LLC KSI Electrical, LLC KSI Mechanical, LLC Land Management & Development, Inc. 3 Landwest, Inc. 3 Massachusetts Electric Company 1 Metro Energy, LLC 3 Metrowest Realty LLC Mystic Steamship Corporation Nantucket Electric Company 1 National Grid Algonquin LLC National Grid Connect Inc. National Grid Development Holdings Corp. National Grid Electric Services, LLC 3 National Grid Energy Management, LLC National Grid Energy Services LLC National Grid Energy Trading Services LLC 3 National Grid Engineering & Survey Inc. 3 National Grid Generation LLC 3 National Grid Generation Ventures LLC 3 National Grid Glenwood Energy Center, LLC National Grid Green Homes Inc. National Grid IGTS Corp. 3 National Grid Insurance USA Ltd 3 National Grid Islander East Pipeline LLC National Grid LNG GP LLC National Grid LNG LLC National Grid LNG LP LLC National Grid Millennium LLC National Grid NE Holdings 2 LLC 1 National Grid North America Inc. National Grid North East Ventures Inc. National Grid Port Jefferson Energy Center LLC National Grid Services Inc. National Grid Technologies Inc. 3 National Grid Transmission Services Corporation 1 National Grid US 6 LLC National Grid US LLC National Grid USA National Grid USA Service Company, Inc. 3 Nees Energy, Inc. 1 New England Electric Transmission Corporation 2 New England Energy Incorporated 1 New England Hydro Finance Company, Inc. ( 53.704% ) 1 New England Hydro-Transmission Corporation ( 53.704% ) 2 New England Hydro-Transmission Electric Company Inc. ( 53.704% ) 1 New England Power Company 1 Newport America Corporation 4 NGNE LLC Niagara Mohawk Energy, Inc. Niagara Mohawk Holdings, Inc. 3 Niagara Mohawk Power Corporation 3 NM Properties, Inc. 3 North East Transmission Co., Inc. Opinac North America, Inc. Philadelphia Coke Co., Inc. Port of the Islands North, LLC 3 The Brooklyn Union Gas Company 3 The Narragansett Electric Company 4 Transgas, Inc. 1 Upper Hudson Development Inc. 1 Valley Appliance and Merchandising Company 4 Vermont Green Line Devco, LLC ( 90% ) Wayfinder Group, Inc. 1 Incorporated in Australia Registered office: Level 7, 330 Collins Street, Melbourne, VIC 3000, Australia National Grid Australia Pty Limited Incorporated in Canada Registered office: 1959 Upper Water Street, Suite 800, Halifax NS, B3J 2X2, Canada KeySpan Energy Development Co. Incorporated in the Cayman Islands Registered office: c/o KPMG, PO Box 493, 2nd Floor, Century Yard, Cricket Square, Grand Cayman KY1-1106, Cayman Islands British Transco Finance (No 1) Limited* British Transco Finance (No 2) Limited* Incorporated in the Isle of Man Registered office: Third Floor, St George’s Court, Upper Church Street, Douglas, IM1 1EE, Isle of Man, UK (unless stated otherwise in footnotes) Lattice Telecom Finance (No 1) Limited 5 * National Grid Insurance Company (Isle of Man) Limited NGT Holding Company (Isle of Man) Limited Incorporated in Jersey Registered office: 44 Esplanade, St Helier, Jersey JE4 9WG, UK National Grid Jersey Investments Limited NG Jersey Limited Incorporated in the Netherlands Registered office: Westblaak 89, 3012 KG Rotterdam, PO Box 21153, 3001 AD, Rotterdam, Netherlands British Transco International Finance B.V. Registered office: Prins Bernhardplein 200, 1097 JB, Amsterdam, Netherlands National Grid Holdings B.V. Incorporated in the Republic of Ireland Registered office: Third Floor, The Metropolitan Building, James Joyce Street, Dublin 1, Ireland National Grid Insurance Company (Ireland) Designated Activity Company 1. Registered office: Corporation Service Company, 84 State Street, Boston MA 02109, Suffolk County, USA. 2. Registered office: Corporation Service Company, 10 Ferry Street, Suite 313, Concord NH 03301, Merrimack County, USA. 3. Registered office: Corporation Service Company, 80 State Street, Albany NY 12207-2543, Albany County, USA. 4. Registered office: Corporation Service Company, 222 Jefferson Boulevard, Suite 200, Warwick RI 02888, Kent County, USA. 5. Registered office: Heritage Court, 41 Athol Street, Douglas, IM99 1HN, Isle of Man, UK. * In liquidation. 32. Subsidiary undertakings, joint ventures and associates continued Joint ventures A list of the Group’s joint ventures as at 31 March 2018 is given below. All joint ventures are included in the Group’s financial statements using the equity method of accounting. Principal joint ventures are identified in bold . Incorporated in England and Wales Registered office: 1–3 Strand, London WC2N 5EH, UK (unless stated otherwise in footnotes). BritNed Development Limited ( 50% )* Joint Radio Company Limited ( 50% ) 1 ** Nemo Link Limited ( 50% ) NGET/SPT Upgrades Limited ( 50% ) † St William Homes LLP ( 50% ) 2 Incorporated in the US Registered office: Corporation Service Company, 251 Little Falls Drive, Wilmington, DE 19808, New Castle County, USA (unless stated otherwise in footnotes). Clean Energy Generation, LLC ( 50% ) Island Park Energy Center, LLC ( 50% ) Islander East Pipeline Company, LLC ( 50% ) 3 LI Energy Storage System, LLC ( 50% ) LI Solar Generation, LLC ( 50% ) Swan Lake North Holdings LLC ( 50% ) Incorporated in France Registered office: 1 Terrasse Bellini, Tour Initiale, TSA 41000 – 9291, Paris La Defense, CEDEX, France IFA2 SAS ( 50% ) Associates A list of the Group’s associates as at 31 March 2018 is given below. All associates are included in the Group’s financial statements using the equity method of accounting. Principal associates are identified in bold . Incorporated in England and Wales Registered office: Ashbrook Court, Prologis Park, Central Boulevard, Coventry CV7 8PE, UK Quadgas HoldCo Limited ( 39% ) Incorporated in the US Registered office: Corporation Service Company, 251 Little Falls Drive, Wilmington, DE 19808, New Castle County, USA (unless stated otherwise in footnotes). Algonquin Gas Transmission, LLC ( 20% ) 3 Clean Line Energy Partners LLC ( 32% ) Connecticut Yankee Atomic Power Company ( 19.5% ) 4 Direct Global Power, Inc. ( 26% ) 5 Energy Impact Fund LP ( 9.7% ) 6 KHB Venture LLC ( 33% ) 7 Maine Yankee Atomic Power Company ( 24% ) 8 Millennium Pipeline Company, LLC ( 26.25% ) 3 New York Transco LLC ( 28.3% ) 9 Nysearch RMLD, LLC ( 22.63% ) Sunrun Neptune Investor 2016 LLC 3 *** Yankee Atomic Electric Company ( 34.5% ) 10 Incorporated in Belgium Registered office: Avenue de Cortenbergh 71, 1000 Brussels, Belgium Coreso SA ( 15.84% ) Other investments A list of the Group’s other investments as at 31 March 2018 is given below. Incorporated in England and Wales Registered office: 1 More London Place, London SE1 2AF, UK Energis plc ( 33.06% ) ‡ 1. Registered office: Friars House, Manor House Drive, Coventry CV1 2TE, UK. 2. Registered office: Berkeley House, 19 Portsmouth Road, Cobham, Surrey KT11 1JG, UK. 3. Registered office: Corporation Trust Company, 1209 Orange, Wilmington DE 19808, New Castle County, USA. 4. Registered office: Carla Pizzella, 362 Injun Hollow Road, East Hampton CT 06424, USA. 5. Registered office: 507 Plum Street, PO Box 5001, Syracuse NY 13250, USA. 6. Registered office: Harvard Business Services, Inc., 16192 Coastal Highway, Lewes DE 19958, Sussex County, USA. 7. Registered office: De Maximus Inc., 135 Beaver Street, 4th Floor, Waltham MA 02452, USA. 8. Registered office: Joseph D Fay, 321 Old Ferry Road, Wiscasset ME 04578, USA. 9. Registered office: Corporation Service Company, 80 State Street, Albany NY 12207, USA. 10. Registered office: Brian Smith, 49 Yankee Road, Rowe MA 01367, USA. * National Grid Interconnector Holdings Limited owns 284,500,000 €0.20 C Ordinary shares and one £1.00 Ordinary A share. ** National Grid Gas plc owns all £1.00 A Ordinary shares. *** National Grid Green Homes Inc owns 1,000 Class A Membership Interests. † National Grid Electricity Transmission plc owns 50 £1.00 A Ordinary shares. ‡ In administration. Our interests and activities are held or operated through the subsidiaries, joint arrangements or associates as disclosed above. These interests and activities (and their branches) are established in – and subject to the laws and regulations of – these jurisdictions.

Derivative financial instrument

Derivative financial instruments	12 Months Ended
	Mar. 31, 2018
Financial Instruments [Abstract]
Derivative financial instruments	Derivative financial instruments Derivatives are financial instruments that derive their value from the price of an underlying item such as interest rates, foreign exchange rates, credit spreads, commodities, equity or other indices. In accordance with Board approved policies, derivatives are transacted generally to manage our exposure to fluctuations in interest rate, foreign exchange and our operational market risks from our commodity activities. Our derivatives are split into the broad categories analysed below: • derivatives managing risks to interest rate and foreign exchange rate. Specifically we use these derivatives to manage risks from the financing portfolio, to optimise the overall cost of accessing the debt capital markets, managing the exposure to holdings in foreign operations and other contractual operational cash flows; and • derivatives managing our price and supply risks from our commodity activity. Derivative financial instruments are initially recognised at fair value and subsequently remeasured to fair value at each reporting date. Changes in fair values are recorded in the period they arise, in either the income statement or other comprehensive income as required by IAS 39. Where the gains or losses recorded in the income statement arise from changes in the fair value of commodity contract derivatives and of derivative financial instruments to the extent that hedge accounting is not achieved or is not effective, these are recorded as remeasurements, detailed in notes 4 and 5. Where the fair value of a derivative is positive it is carried as a derivative asset, and where negative as a derivative liability. The total fair value of derivatives split by category is as follows: 2018 2017 Assets £m Liabilities £m Total £m Assets £m Liabilities £m Total £m Financing derivatives 1,545 (945 ) 600 1,707 (2,223 ) (516 ) Commodity derivatives 1 69 (116 ) (47 ) 106 (170 ) (64 ) Further Acquisition Agreement derivative 2 110 — 110 — — — 1,724 (1,061 ) 663 1,813 (2,393 ) (580 ) 1. Comparative amounts have been re-presented to reflect the reclassification of commodity derivative contracts from trade and other receivables ( 31 March 2017 : £54 million ), trade and other payables ( 31 March 2017 : £93 million ), other non-current assets ( 31 March 2017 : £52 million ) and other non-current liabilities ( 31 March 2017 : £77 million ), to current and non-current derivative financial assets and derivative financial liabilities. 2. This year a further derivative category has been added for the Further Acquisition Agreement (FAA) derivative. This relates to the put/call option over a 14% interest in Quadgas HoldCo Limited. Refer to note 15 for further details. (a) Financing derivatives We calculate fair value of the financial derivatives by discounting all future cash flows using the market yield curve at the reporting date. The market yield curve for each currency is obtained from external sources for interest and foreign exchange rates. In the absence of sufficient market data, fair values would be based on the quoted market price of similar derivatives. Analysis of these derivatives and the various methods used to calculate their respective fair values is detailed below and in note 30. For each class of financing derivative instrument type the fair value amounts are as follows: 2018 2017 Assets £m Liabilities £m Total £m Assets £m Liabilities £m Total £m Interest rate swaps 678 (457 ) 221 849 (657 ) 192 Cross-currency interest rate swaps 687 (207 ) 480 676 (909 ) (233 ) Foreign exchange forward contracts 1 174 (2 ) 172 160 (113 ) 47 Inflation linked swaps 5 (278 ) (273 ) 7 (529 ) (522 ) Equity options 1 (1 ) — 15 (15 ) — 1,545 (945 ) 600 1,707 (2,223 ) (516 ) 1. Included within the foreign exchange forward contracts balance is £67 million ( 2017 : £69 million ) of derivatives in relation to hedging of capital expenditure. The maturity profile of financing derivative instruments is as follows: 2018 2017 Assets £m Liabilities £m Total £m Assets £m Liabilities £m Total Current Less than 1 year 375 (325 ) 50 192 (1,054 ) (862 ) 375 (325 ) 50 192 (1,054 ) (862 ) Non-current In 1 to 2 years 83 (88 ) (5 ) 199 (305 ) (106 ) In 2 to 3 years 25 (27 ) (2 ) 122 (160 ) (38 ) In 3 to 4 years 418 (5 ) 413 39 (83 ) (44 ) In 4 to 5 years 12 — 12 419 (36 ) 383 More than 5 years 632 (500 ) 132 736 (585 ) 151 1,170 (620 ) 550 1,515 (1,169 ) 346 1,545 (945 ) 600 1,707 (2,223 ) (516 ) 16. Derivative financial instruments continued (a) Financing derivatives continued For each class of financing derivative the notional contract 1 amounts are as follows: 2018 2017 £m £m Interest rate swaps (8,390 ) (9,469 ) Cross-currency interest rate swaps (6,925 ) (8,631 ) Foreign exchange forward contracts (5,793 ) (8,253 ) Inflation linked swaps (1,191 ) (1,423 ) Equity options (800 ) (800 ) (23,099 ) (28,576 ) 1. The notional contract amounts of derivatives indicate the gross nominal value of transactions outstanding at the reporting date. (b) Commodity risk We purchase electricity and gas to supply our customers in the US and to meet our own energy needs. Substantially all our costs of purchasing electricity and gas for supply to customers are recoverable at an amount equal to cost. The timing of recovery of these costs can vary between financial periods leading to an under- or over-recovery within any particular year that can lead to large fluctuations in the income statement. We follow approved policies to manage price and supply risks for our commodity activities. Our energy procurement risk management policy and delegations of authority govern our US commodity trading activities for energy transactions. The purpose of this policy is to ensure we transact within pre-defined risk parameters and only in the physical and financial markets where we or our customers have a physical market requirement. In addition, state regulators require National Grid to manage commodity risk and cost volatility prudently through diversified pricing strategies. In some jurisdictions we are required to file a plan outlining our strategy to be approved by regulators. In certain cases we might receive guidance with regard to specific hedging limits. Energy purchase contracts for the forward purchase of electricity or gas that are used to satisfy physical delivery requirements to customers, or for energy that the Group uses itself, meet the expected purchase or usage requirements of IAS 39. They are, therefore, not recognised in the financial statements until they are realised. Disclosure of commitments under such contracts is made in note 28. US states have introduced a variety of legislative requirements with the aim of increasing the proportion of our electricity that is derived from renewable or other forms of clean energy. Annual compliance filings regarding the level of Renewable Energy Certificates (and other similar environmental certificates) are required by the relevant department of utilities. In response to the legislative requirements, National Grid has entered into long-term, typically fixed-price, energy supply contracts to purchase both renewable energy and environmental certificates. We are entitled to recover all costs incurred under these contracts through customer billing. Under IFRS, where these supply contracts are not accounted for as finance leases, they are considered to comprise two components, being a forward purchase of power at spot prices, and a forward purchase of environmental certificates at a variable price (being the contract price less the spot power price). With respect to our current contracts, neither of these components meets the requirement to be accounted for as a derivative. The environmental certificates are currently required for compliance purposes, and at present there are no liquid markets for these attributes. Accordingly, this component meets the expected purchase or usage requirements of IAS 39. We expect to enter into an increasing number of these contracts, in order to meet our compliance requirements in the short to medium term. It is possible that in future, if and when liquid markets develop, and to the extent that we are in receipt of environmental certificates in excess of our required levels, this exemption may cease to apply and we may be required to account for forward purchase commitments for environmental certificates as derivatives at fair value through profit and loss. Some forward contracts for the purchase of commodities do not meet the exemption for accounting purposes and hence are accounted for as derivatives. We also enter into derivative financial instruments linked to commodity prices, including index-linked futures, swaps, and option contracts. These derivative financial instruments are used to manage market price volatility and are carried at fair value on the statement of financial position, with the mark-to-market changes reflected through remeasurements. For each class of commodity contract derivative type, the fair value amounts are as follows: 2018 2017 Assets £m Liabilities £m Total £m Assets Liabilities £m Total £m Commodity purchase contracts accounted for as derivative contracts Forward purchases of electricity — — — — (10 ) (10 ) Forward purchases of gas 60 (64 ) (4 ) 82 (97 ) (15 ) Derivative financial instruments linked to commodity prices Electricity capacity 1 — 1 2 — 2 Electricity swaps 7 (46 ) (39 ) 11 (61 ) (50 ) Electricity options — (1 ) (1 ) — — — Gas swaps 1 (4 ) (3 ) 11 (2 ) 9 Gas options — (1 ) (1 ) — — — 69 (116 ) (47 ) 106 (170 ) (64 ) 16. Derivative financial instruments continued (b) Commodity risk continued The maturity profile of commodity contracts is as follows: 2018 2017 Assets £m Liabilities £m Total £m Assets £m Liabilities £m Total £m Current Less than one year 30 (76 ) (46 ) 54 (93) (39) 30 (76 ) (46 ) 54 (93) (39) Non-current In 1 to 2 years 6 (17 ) (11 ) 8 (36) (28) In 2 to 3 years 6 (11 ) (5 ) 7 (9) (2) In 3 to 4 years 6 (3 ) 3 6 (7) (1) In 4 to 5 years 5 (2 ) 3 6 (5) 1 More than 5 years 16 (7 ) 9 25 (20) 5 39 (40 ) (1 ) 52 (77) (25) 69 (116 ) (47 ) 106 (170) (64) For each class of commodity contract derivative, our exposure based on the notional quantities is as follows: 2018 2017 Forward purchases of electricity 1 0 GWh 159 GWh Forward purchases of gas 2 54m Dth 54m Dth Electricity swaps 12,839 GWh 12,776 GWh Electricity options 13,897 GWh 17,793 GWh Electricity capacity 0.6 GWm 0.7 GWm Gas swaps 100m Dth 83m Dth Gas options 7m Dth 9m Dth NYMEX gas futures 3 0 Dth 3m Dth 1. Forward electricity purchases expired on 30 September 2017. The contractual obligations under these contracts are £nil ( 2017 : £15 million ). 2. Forward gas purchases have terms up to three years . The contractual obligations under these contracts are £96 million ( 2017 : £131 million ). 3. NYMEX gas futures have been offset with related margin accounts (see note 30(a)). (c) Further Acquisition Agreement derivative The Group signed a Further Acquisition Agreement (FAA) concerning a 14% interest in Quadgas HoldCo Limited, which contains put and call options. The fair value of these derivatives is £110 million asset at 31 March 2018 ( 2017 : £nil ). The derivative notional was £739 million ( 2017 : £690 million ). The options can be exercised in the period between 1 March 2019 and 31 October 2019. The fair value of the option has been disclosed as a non-current derivative asset. (d) Hedge accounting Where possible, derivatives held as hedging instruments are formally designated as hedges as defined in IAS 39. Derivatives may qualify as hedges for accounting purposes if they are fair value hedges, cash flow hedges or net investment hedges. Our use of derivatives may entail a derivative transaction qualifying for one or more hedge type designations under IAS 39. Hedge accounting allows derivatives to be designated as a hedge of another non-derivative financial instrument, to mitigate the impact of potential volatility in the income statement of changes in the fair value of the derivative instruments. To qualify for hedge accounting, documentation is prepared specifying the hedging strategy, the component transactions and methodology used for effectiveness measurement. National Grid uses three hedge accounting methods, which are described as follows: Fair value hedges Fair value hedges principally consist of interest rate and cross-currency swaps that are used to protect against changes in the fair value of fixed-rate, long-term financial instruments due to movements in market interest rates and foreign exchange rates. For qualifying fair value hedges, all changes in the fair value of the derivative and changes in the fair value of the item in relation to the risk being hedged are offset in the income statement to the extent the fair value hedge is effective. Adjustments made to the carrying amount of the hedged item for fair value hedges will be amortised over the remaining life, in line with the hedged item. 2018 2017 £m £m Cross-currency interest rate/interest rate swaps 496 548 16. Derivative financial instruments continued (d) Hedge accounting continued Cash flow hedges Exposure arises from the variability in future interest and currency cash flows on assets and liabilities which bear interest at variable rates or are in a foreign currency. Interest rate and cross-currency swaps are maintained, and designated as cash flow hedges, where they qualify, to manage this exposure. Fair value changes on designated cash flow hedges are initially recognised in other comprehensive income as gains or losses recognised in equity and transferred to the cash flow hedge reserve, and any ineffective portion is recognised immediately in the income statement. Amounts are transferred from equity and recognised in the income statement as the income or expense is recognised on the hedged item. Forward foreign currency contracts are used to hedge anticipated and committed future currency cash flows. Where these contracts qualify for hedge accounting they are designated as cash flow hedges. On recognition of the underlying transaction in the financial statements, the associated hedge gains and losses, deferred in equity, are transferred and included with the recognition of the underlying transaction. When a forecast transaction is no longer expected to occur, the cumulative gain or loss previously reported in equity is transferred to the income statement. Where a non-financial asset or a non-financial liability results from a forecast transaction or firm commitment being hedged, the amounts deferred in equity are included in the initial measurement of that non-monetary asset or liability. 2018 2017 £m £m Cross-currency interest rate/interest rate swaps 138 (180 ) Foreign exchange forward contracts 66 69 204 (111 ) Net investment hedges Borrowings, cross-currency swaps and forward currency contracts are used in the management of the foreign exchange exposure arising from the investment in non-sterling denominated subsidiaries. Where these contracts qualify for hedge accounting they are designated as net investment hedges. 2018 2017 £m £m Cross-currency interest rate/interest rate swaps (104 ) (544 ) Foreign exchange forward contracts 87 (56 ) (17 ) (600 ) The cross-currency swaps and forward foreign currency contracts are hedge accounted using the spot to spot method. The foreign exchange gain or loss on retranslation of the borrowings and the spot to spot movements on the cross-currency swaps and forward currency contracts are transferred to equity to offset gains or losses on translation of the net investment in the non-sterling denominated subsidiaries, with any ineffective portion recognised immediately in the income statement. Derivatives not in a formal hedge relationship Our policy is not to use derivatives for trading purposes. However, due to the complex nature of hedge accounting under IAS 39 some derivatives may not qualify for hedge accounting, or are specifically not designated as a hedge where natural offset is more appropriate. Changes in the fair value of any derivative instruments that do not qualify for hedge accounting are recognised in remeasurements within the income statement. 2018 2017 £m £m Cross-currency interest rate/interest rate swaps 171 135 Foreign exchange forward contracts 19 34 Inflation linked swaps (273 ) (522 ) Equity options — — Commodity derivatives (all categories) (47 ) (64 ) Further Acquisition Agreement derivative 110 — (20 ) (417 ) Discontinuation of hedge accounting Hedge accounting is discontinued when the hedging instrument expires or is sold, terminated, exercised or no longer qualifies for hedge accounting. At that time, any cumulative gains or losses relating to cash flow hedges recognised in equity are initially retained in equity and subsequently recognised in the income statement in the same periods in which the previously hedged item affects profit or loss, unless the hedged item is no longer expected to occur and then the amounts would be recognised immediately. Amounts deferred in equity with respect to net investment hedges are subsequently recognised in the income statement in the event of the disposal of the overseas operations concerned. For fair value hedges, the cumulative adjustment recorded to the carrying value of the hedged item at the date hedge accounting is discontinued is amortised to the income statement using the effective interest method. Embedded derivatives No adjustment is made with respect to derivative clauses embedded in financial instruments or other host contracts that are defined as closely related to those instruments or contracts. Consequently these embedded derivatives are not accounted for separately from the debt instrument. Where there are embedded derivatives in host contracts that are not closely related, the embedded derivative is separately accounted for as a derivative financial instrument.

Inventories and other current i

Inventories and other current intangibles	12 Months Ended
	Mar. 31, 2018
Subclassifications of assets, liabilities and equities [abstract]
Inventories and current intangible assets	Inventories and current intangible assets Inventories represent assets that we intend to use in order to generate revenue in the short-term, either by selling the asset itself (for example, fuel stocks) or by using it to fulfil a service to a customer or to maintain our network (consumables). Inventories are stated at the lower of weighted average cost and net realisable value. Where applicable, cost comprises direct materials and direct labour costs as well as those overheads that have been incurred in bringing the inventories to their present location and condition. Emission allowances, principally relating to the emissions of carbon dioxide in the UK and sulphur and nitrous oxides in the US, are recorded as intangible assets within current assets, and are initially recorded at cost and subsequently at the lower of cost and net realisable value. A liability is recorded in respect of the obligation to deliver emission allowances, and emission charges are recognised in the income statement in the period in which emissions are made. 2018 2017 £m £m Fuel stocks 78 101 Raw materials and consumables 190 191 Work in progress — 8 Current intangible assets – emission allowances 73 103 341 403 There is a provision for obsolescence of £18 million against inventories as at 31 March 2018 ( 2017 : £26 million ).

Trade and other receivables

Trade and other receivables	12 Months Ended
	Mar. 31, 2018
Trade and other receivables [abstract]
Trade and other receivables	Trade and other receivables Trade and other receivables are amounts which are due from our customers for services we have provided. Other receivables also include prepayments made by us, for example, property lease rentals paid in advance. Trade and other receivables are initially recognised at fair value and subsequently measured at amortised cost, less any appropriate allowances for estimated irrecoverable amounts. A provision is established for irrecoverable amounts when there is objective evidence that amounts due under the original payment terms will not be collected. 2018 2017¹ £m £m Trade receivables 1,674 1,591 Accrued income 817 811 Prepayments 229 228 Other receivables 78 98 2,798 2,728 1. Comparative amounts have been represented to reflect the reclassification of commodity contract derivatives from trade and other receivables to derivatives (see note 16). Trade receivables are non-interest-bearing and generally have a 30 to 90 days term. Due to their short maturities, the fair value of trade and other receivables approximates their book value. Provision for impairment of receivables 2018 2017 £m £m At 1 April 424 349 Exchange adjustments (42 ) 51 Charge for the year, net of recoveries 36 147 Uncollectible amounts written off against receivables (109 ) (121 ) Disposal of UK Gas Distribution — (2 ) At 31 March 309 424 Trade receivables past due but not impaired 2018 2017 £m £m Up to 3 months past due 271 238 3 to 6 months past due 73 67 Over 6 months past due 131 143 475 448 For further information on our wholesale and retail credit risk, refer to note 30(a).

Cash and cash equivalents

Cash and cash equivalents	12 Months Ended
	Mar. 31, 2018
Cash and cash equivalents [abstract]
Cash and cash equivalents	Cash and cash equivalents Cash and cash equivalents include cash balances, together with short-term investments with an original maturity of less than three months that are readily convertible to cash. Net cash and cash equivalents reflected in the cash flow statement are net of bank overdrafts, which are reported in borrowings. The carrying amounts of cash and cash equivalents and bank overdrafts approximate their fair values. Cash at bank earns interest at floating rates based on daily bank deposit rates. Short-term deposits are made for periods varying between one day and three months, depending on the immediate cash requirements, and earn interest at the respective short-term deposit rates. Net cash and cash equivalents held in currencies other than sterling have been converted into sterling at year-end exchange rates. For further information on currency exposures, refer to note 30(d). 2018 2017 £m £m Cash at bank 54 199 Short-term deposits 275 940 Cash and cash equivalents 329 1,139

Borrowings

Borrowings	12 Months Ended
	Mar. 31, 2018
Disclosure of detailed information about borrowings [abstract]
Borrowings	Borrowings We borrow money primarily in the form of bonds and bank loans. These are for a fixed term and may have fixed or floating interest rates or are linked to RPI. As indicated in note 16, we use derivatives to manage risks associated with interest rates and foreign exchange. Further details on our net debt can be found in note 27. Our strategy in action Our price controls and rate plans require us to fund our networks within a certain ratio of debt to equity and, as a result, we have issued a significant amount of debt. As we continue to invest in our networks, the value of debt is expected to increase over time. To maintain a strong balance sheet and to allow us to access capital markets at commercially acceptable interest rates, we balance the amount of debt we issue with the value of our assets, and take account of certain other metrics used by credit rating agencies. Borrowings, which include interest-bearing and inflation-linked debt and overdrafts, are recorded at their initial fair values which normally reflect the proceeds received, net of direct issue costs less any repayments. Subsequently these are stated at amortised cost, using the effective interest method. Any difference between the proceeds after direct issue costs and the redemption value is recognised over the term of the borrowing in the income statement using the effective interest method. 2018 2017 £m £m Current Bank loans 2,020 1,339 Bonds 2,156 2,209 Commercial paper 206 1,881 Finance leases 64 66 Other loans 1 1 4,447 5,496 Non-current Bank loans 2,384 2,343 Bonds 19,418 20,368 Finance leases 207 242 Other loans 169 189 22,178 23,142 Total borrowings 26,625 28,638 20. Borrowings continued Total borrowings are repayable as follows: 2018 2017 £m £m Less than 1 year 4,447 5,496 In 1 to 2 years 1,694 1,941 In 2 to 3 years 2,347 1,821 In 3 to 4 years 1,873 2,453 In 4 to 5 years 1,469 1,921 More than 5 years: By instalments 1,010 1,043 Other than by instalments 13,785 13,963 26,625 28,638 The fair value of borrowings at 31 March 2018 was £30,164 million ( 2017 : £32,239 million ). Where market values were available, fair value of borrowings (Level 1) was £13,018 million ( 2017 : £12,547 million ). Where market values were not available, fair value of borrowings (Level 2) was £17,146 million ( 2017 : £19,692 million ), calculated by discounting cash flows at prevailing interest rates. The notional amount outstanding of the debt portfolio at 31 March 2018 was £26,363 million ( 2017 : £28,310 million ). The assets of the Colonial Gas Company and the Niagara Mohawk Power Corporation and certain gas distribution assets of the Narragansett Electric Company are subject to liens and other charges and are provided as collateral over borrowings totalling £392 million at 31 March 2018 ( 2017 : £440 million ). Collateral is placed with or received from any counterparty where we have entered into a credit support annex to the ISDA Master Agreement once the current mark-to-market valuation of the trades between the parties exceeds an agreed threshold. Included in current bank loans is £878 million ( 2017 : £709 million ) in respect of cash received under collateral agreements. For further details of our borrowing facilities, refer to note 31. For further details of our bonds in issue, please refer to the debt investor section of our website. Unless included herein, the information on our website is unaudited. Assets held under finance leases are recognised at their fair value or, if lower, the present value of the minimum lease payments on inception. The corresponding liability is recognised as a finance lease obligation within borrowings. Rental payments are apportioned between finance costs and reduction in the finance lease obligation, so as to achieve a constant rate of interest. Assets held under finance leases are depreciated over the shorter of their useful life and the lease term. Finance lease obligations 2018 2017 £m £m Gross finance lease liabilities are repayable as follows: Less than 1 year 64 66 1 to 5 years 177 213 More than 5 years 72 89 313 368 Less: finance charges allocated to future periods (42 ) (60 ) 271 308 The present value of finance lease liabilities is as follows: Less than 1 year 64 66 1 to 5 years 144 174 More than 5 years 63 68 271 308 Unaudited commentary on borrowings As at 31 March 2018 , total borrowings of £26,625 million ( 2017 : £28,638 million ) including bonds, bank loans, commercial paper, collateral, finance leases and other debt had decreased by £2,013 million . We expect to repay £4,447 million of our total borrowings in the next 12 months including commercial paper, collateral and interest, and to fund this repayment through the capital and money markets and surplus cash. The average long-term debt maturity of the portfolio is 11 years ( 2017 : 11 years ). Further information on our bonds can be found in the debt investor section of our website. Unless included herein, the information on our website is unaudited.

Trade and other payables

Trade and other payables	12 Months Ended
	Mar. 31, 2018
Subclassifications of assets, liabilities and equities [abstract]
Trade and other payables	Trade and other payables Trade and other payables include amounts owed to suppliers, tax authorities and other parties which are due to be settled within 12 months. The total also includes deferred income, which represents monies received from customers but for which we have not yet delivered the associated service. These amounts are recognised as revenue when the service is provided. Trade and other payables are initially recognised at fair value and subsequently measured at amortised cost. 2018 2017¹ £m £m Trade payables 1,977 2,135 Deferred income 2 440 298 Social security and other taxes 173 136 Other payables 863 776 3,453 3,345 1. Comparative amounts have been represented to reflect the reclassification of commodity derivative contracts from trade and other payables to derivatives (see note 16). 2. Included within deferred income is £85 million ( 2017 : £89 million ) relating to customer contributions. Due to their short maturities, the fair value of trade payables approximates their book value.

Other non-current liabilities

Other non-current liabilities	12 Months Ended
	Mar. 31, 2018
Subclassifications of assets, liabilities and equities [abstract]
Other non-current liabilities	Other non-current liabilities Other non-current liabilities include deferred income which will not be recognised as income until after 31 March 2019. It also includes payables that are not due until after that date. Non-current liabilities are initially recognised at fair value and subsequently measured at amortised cost. 2018 2017¹ £m £m Deferred income 2 958 1,032 Other payables 359 338 1,317 1,370 1. Comparative amounts have been represented to reflect the reclassification of commodity derivative contracts from other non-current liabilities to derivatives (see note 16). 2. Included within deferred income is £844 million ( 2017 : £839 million ) relating to customer contributions. There is no material difference between the fair value and the carrying value of other payables.

Pensions and other post retirem

Pensions and other post retirement benefits	12 Months Ended
	Mar. 31, 2018
Employee Benefits [Abstract]
Pensions and other post retirement benefits	Pensions and other post-retirement benefits The majority of employees are either members of a DB (defined benefit) or a DC (defined contribution) pension scheme. In the US we also provide healthcare and life insurance benefits to eligible retired US employees. The fair value of associated scheme assets and present value of DB obligations are updated annually in accordance with IAS 19 (revised). We separately present our UK and US pension schemes to show geographical split. Below we provide a more detailed analysis of the amounts recorded in the primary financial statements and the actuarial assumptions used to value the DB obligations. Defined contribution plans Most new hires are able to join DC plans in the UK or US. These plans are designed to provide members with a pension pot for their retirement. The risks associated with these plans are assumed by the member. Payments to these DC plans are charged as an expense as they fall due. There is no legal or constructive obligation on National Grid to pay additional contributions into a DC plan if the fund has insufficient assets to pay all employees’ benefits relating to employee service in the current and prior periods. Defined benefit schemes The principal UK schemes are the National Grid UK Pension Scheme (NGUKPS) and the National Grid Electricity Group of the Electricity Supply Pension Scheme (NGEG of the ESPS). In the US, we have four plans and a number of healthcare and life insurance plans. For DB pension schemes, members receive benefits on retirement, the value of which is dependent on factors such as salary and length of pensionable service. National Grid underwrites both financial and demographic risks associated with this type of scheme. National Grid’s obligation in respect of DB pension schemes is calculated separately for each DB scheme by projecting the estimated amount of future benefit payments that employees have earned for their pensionable service in the current and prior periods. These future benefit payments are discounted to determine the present value of the liabilities. Current service cost and any unrecognised past service cost are recognised immediately. The discount rate used is the yield at the valuation date on high-quality corporate bonds. Advice is taken from independent actuaries relating to the appropriateness of the key assumptions applied, including life expectancy, expected salary and pension increases, and inflation. Comparatively small changes in the assumptions used may have a significant effect on the amounts recognised in the income statement, the statement of other comprehensive income and the net liability recognised in the statement of financial position. Remeasurements of pension assets and post-retirement benefit obligations are recognised in full in the period in which they occur in the statement of other comprehensive income. 23. Pensions and other post-retirement benefits continued UK pension plans Defined Contribution The National Grid YouPlan National Grid pays contributions into YouPlan to provide DC benefits on behalf of employees. National Grid provides a double match of member contributions, up to a maximum of 6% of member salary. YouPlan was established in 2013 and is the qualifying scheme used for automatic enrolment of new hires. Prior to its establishment, DC benefits were provided through the NGUKPS, which transferred all historical DC benefits to YouPlan in 2013. Defined Benefit National Grid’s DB pension arrangements are held in separate trustee administered funds. The arrangements are managed by trustee companies with boards consisting of company and member appointed directors. The arrangements are subject to independent actuarial funding valuations at least every three years and following consultation and agreement with us, the qualified actuary certifies the employers’ contributions, which, together with the specified contributions payable by the employees and proceeds from the plans’ assets, are expected to be sufficient to fund the benefits payable. The results of the most recent actuarial valuations are shown below. See page 145 for the assumptions used for IAS 19 purposes. Section A of NGUKPS Section B of NGUKPS NGEG of ESPS Latest full actuarial valuation 31 March 2017 31 March 2017 31 March 2016 Actuary Willis Towers Watson Willis Towers Watson Aon Hewitt Market value of scheme assets at latest valuation £6,716 m £5,849 m £2,553 m Actuarial value of benefits due to members £6,627 m £6,057 m £3,053 m Market value as percentage of benefits 101% 97% 84% Funding surplus/(deficit) £89 m (£208m) (£500m) Funding surplus/(deficit) net of tax £74 m (£173m) (£415m) National Grid UK Pension Scheme With effect from 1 January 2017 the scheme was split into three sections, each of which is legally and actuarially separate. Sections A and B are supported by companies within the National Grid Group, and National Grid makes payments to Sections A and B to cover administration costs and the Pension Protection Fund levy. The scheme closed to new hires on 1 April 2002. Section C is supported by Cadent Gas Limited which is no longer part of the National Grid Group following the disposal of UK Gas Distribution which completed on 31 March 2017. The disclosures for the current year do not include Section C data. However, the actuarial assumptions adopted will influence our equity accounting for Quadgas HoldCo Limited (the ultimate parent of Cadent Gas Limited). Section A Following the actuarial valuation at 31 March 2017, Section A was in surplus. As such, no deficit funding contributions are being made to the section. National Grid and the Trustees have agreed a schedule of contributions whereby the employers will contribute 51.8% of pensionable salary, less member contributions, in respect of ongoing service cost. This rate is deemed to be sufficient to meet the statutory funding objective during the period for which it is in force. The next actuarial valuation will be performed at 31 March 2019. Section B The actuarial valuation as at 31 March 2017 determined that Section B was in deficit. National Grid and the Trustee agreed on a schedule of contributions, whereby deficit funding contributions of approximately £32 million payable by 30 September each year from 2017 onwards until 30 September 2022, with an additional £8 million payable by 30 September 2023. All deficit funding amounts due will be adjusted for the change in the Retail Price Index (RPI) from 31 December 2016 up to three months prior to the date of payment. As a result, the funding shortfall is expected to be eliminated by 30 September 2023. The employers will also contribute 51.4% of pensionable salary, less member contributions, in respect of ongoing service cost. The next actuarial valuation will be performed as at 31 March 2019. 23. Pensions and other post-retirement benefits continued National Grid Electricity Group of the Electricity Supply Pension Scheme The scheme closed to new hires from 1 April 2006. The last full actuarial valuation for the NGEG of the ESPS was carried out at 31 March 2016 and determined that the scheme was in deficit. National Grid and the Trustee agreed on a schedule of contribution, whereby deficit funding contributions of £48 million are payable each year from 2016 onwards until 31 March 2027. All deficit funding amounts due will be adjusted for the change in the Retail Price Index (RPI). The funding shortfall is expected to be eliminated by 31 March 2027. The employer will also contribute 40.7% of pensionable earnings (less member contributions). The next actuarial valuation is required with an effective date of no later than 31 March 2019. Security arrangements National Grid has also established security arrangements with charges in favour of the Trustees. As at 31 March 2018 the required value of these arrangements and principal supporting employers were: • Section A of NGUKPS: £315 million , National Grid plc and National Grid UK Limited; • Section B of NGUKPS: £179 million , National Grid Gas plc (NGG); and • NGEG of ESPS: £250 million , National Grid Electricity Transmission plc (NGET). The majority of the security is provided in the form of letters of credit with the remainder in surety bonds. The assets held in security will be paid to the respective section or scheme in the event that the relevant supporting employer is subject to an insolvency event or fails to make the required contributions. In addition, counter indemnities have also been taken out to ensure the obligations will be fulfilled. The following specific conditions apply to the security arrangements: • Section B of NGUKPS: The assets will be paid to Section B if NGG is given notice of less than 12 months that Ofgem intends to revoke its licence under the Gas Act 1986, or if NGG grants any charges over its assets other than where agreed with the Trustee. • NGEG of ESPS: The asset and an amount in respect of the deficit (to a maximum of £500 million ) will be paid to the scheme if NGET ceases to hold a licence granted under the Electricity Act 1989. In addition to the above, if the credit rating of the supporting employer, as determined by two out of three specified agencies, falls below certain agreed levels for 40 days , the following amounts will be payable: • Section A of NGUKPS: £72 million (increased in line with RPI). • Section B of NGUKPS: £65 million (increased in line with RPI). • NGEG of ESPS: A maximum of £500 million in respect of the deficit. US pension plans National Grid has multiple DC pension plans, primarily comprised of employee savings and Company matching contributions. Non-union employees hired after 1 January 2011, as well as new hires in the majority of represented union employees, receive a core contribution into the DC plan, irrespective of the employee’s contribution into the plan. National Grid also sponsors numerous non-contributory DB pension plans. The DB plans provide retirement benefits to vested union employees, as well as vested non-union employees hired before 1 January 2011. Benefits under these plans generally reflect age, years of service and compensation and are paid in the form of an annuity or lump sum. An independent actuary performs valuations annually. The Company funds the DB plans by contributing no less than the minimum amount required, but no more than the maximum tax deductible amount allowed under US Internal Revenue Service regulations. The range of contributions determined under these regulations can vary significantly depending upon the funded status of the plans. At present, there is some flexibility in the amount that is contributed on an annual basis. In general, the Company’s policy for funding the US pension plans is to contribute the amounts collected in rates and capitalised in the rate base during the year, to the extent that the funding is no less than the minimum amount required. The assets of the plans are held in trusts and administered by fiduciary committees comprised of appointed employees of the Company. US retiree healthcare and life insurance plans National Grid provides healthcare and life insurance benefits to eligible retired US employees. Eligibility is based on certain age and length of service requirements and in most cases retirees contribute to the cost of their healthcare coverage. In the US, there is no governmental requirement to pre-fund post-retirement healthcare and life insurance plans. However, in general, the Company’s policy for funding the US retiree healthcare and life insurance plans is to contribute amounts collected in rates and capitalised in the rate base during the year. 23. Pensions and other post-retirement benefits continued Risks National Grid underwrites the financial and demographic risks associated with DB plans, the most significant of which are: • Asset volatility: The schemes invest in a variety of asset classes, principally: equities, government securities, corporate bonds and property. Consequently actual returns will differ from the underlying discount rate adopted, impacting on the net balance sheet asset or liability. Each scheme seeks to balance the level of investment return sought with the aim of reducing volatility and risk by means of liability matching asset strategies, diversification of asset portfolios, interest rate hedging and management of foreign exchange exposure. In taking this approach, reference is made to both the maturity of liabilities and the funding level of that scheme. • Changes in bond yields: Liabilities are calculated using discount rates set with reference to yields on high-quality corporate bonds prevailing in the US and UK debt markets and will fluctuate as yields change. • Inflation risk: Changes in inflation will affect current and future pensions, but are partially mitigated though investing in inflation matching assets and hedging instruments. • Member longevity: Longevity is a key driver of liabilities and changes in expected mortality will have a direct impact on liabilities. In aggregate, the liabilities are relatively mature which mitigates the risk to a certain extent. The NGEG of ESPS holds a longevity insurance contract which covers exposure to improvements in longevity, providing long-term protection and income to the scheme in the event that members live longer than currently expected. Actuarial assumptions The Company has applied the following financial assumptions in assessing DB liabilities. UK pensions 2018 2017 2016 % % % Discount rate 1 – past service 2 2.60 2.40 3.30 Discount rate 1 – future service 2 2.65 2.65 3.30 Rate of increase in salaries 3 3.40 3.50 3.20 Rate of increase in RPI – past service 4 3.15 3.20 2.90 Rate of increase in RPI – future service 4 3.10 3.15 2.90 1. The discount rates for pension liabilities have been determined by reference to appropriate yields on high-quality corporate bonds prevailing in the UK debt markets at the reporting date. 2. In the UK for 2018, National Grid has adopted a different discount rate assumption by increasing the duration of the scheme liabilities to 25 years for future service obligations. This has led to a future service discount rate in the UK of 2.65% for both the 2017 and 2018 year-ends. The 2017 discount rate was 2.40% based on an expected duration of scheme liabilities of 17 years . 3. A promotional scale has also been used where appropriate. The UK assumption stated is that relating to service prior to 1 April 2013. The UK assumption for the rate of increase in salaries for past service after this date is 2.20% (2017: 2.20% ; 2016: 2.10% ) and for future service 2.15% (2017: 2.20% ; 2016: 2.10% ). The rates of increase stated are not indicative of historical increases awarded or a guarantee of future increases, but merely an appropriate assumption utilised in assessing DB liabilities. 4. This is the key assumption that determines assumed increases in pensions in payment and deferment in the UK only. Consistent with the derivation of the discount rate, the RPI assumption reflects the duration of the active liabilities to be adopted in the calculation of the future service obligations. This approach leads to a RPI assumption for the future service rate of 3.10% p.a. at reporting date (2017: 3.15% ; 2016: 2.90% ), as compared to the 2017 published assumption of 3.20% for both past service and future service. US pensions US other post-retirement benefits 2018 2017 2016 2018 2017 2016 % % % % % % Discount rate 1 4.00 4.25 4.25 4.00 4.25 4.25 Rate of increase in salaries 3.50 3.50 3.50 3.50 3.50 3.50 Initial healthcare cost trend rate n/a n/a n/a 7.50 7.00 7.50 Ultimate healthcare cost trend rate 2 n/a n/a n/a 4.50 4.50 4.50 1. The discount rates for pension liabilities have been determined by reference to appropriate yields on high-quality corporate bonds prevailing in the US debt markets at the reporting date with an expected duration of plan liabilities of 17 years . 2. The ultimate healthcare cost trend rate will reach the ultimate trend in 2026/28 (2017 and 2016: 2024/25). For sensitivity analysis see note 33. 2018 2017 2016 UK years US years UK years US years UK years US years Assumed life expectations for a retiree age 65 Males 22.3 22.0 22.9 21.9 22.8 21.8 Females 23.9 24.2 24.7 24.1 25.2 24.0 In 20 years: Males 23.7 23.6 25.1 23.6 25.1 23.5 Females 25.5 25.8 27.1 25.7 27.6 25.6 Maturity profile of DB obligations The weighted average duration of the DB obligation for each category of scheme is 16 years for UK pension schemes; 13 years for US pension schemes and 15 years for US other post-retirement benefits. 23. Pensions and other post-retirement benefits continued Amounts recognised in the statement of financial position 2018 2017 2016 £m £m £m Present value of funded obligations 1 (23,747 ) (25,890 ) (28,648 ) Fair value of plan assets 23,858 24,375 26,434 111 (1,515 ) (2,214 ) Present value of unfunded obligations (307 ) (340 ) (304 ) Other post-employment liabilities (67 ) (78 ) (67 ) Net defined benefit liability (263 ) (1,933 ) (2,585 ) Represented by: Liabilities (1,672 ) (2,536 ) (2,995 ) Assets 1,409 603 410 (263 ) (1,933 ) (2,585 ) The geographical split of pensions and other post-retirement benefits is as shown below: UK Pensions US Pensions US other post-retirement benefits 2018 2017 2016 2018 2017 2016 2018 2017 2016 £m £m £m £m £m £m £m £m £m Present value of funded obligations 1 (14,152 ) (15,565 ) (19,341 ) (6,349 ) (6,790 ) (5,916 ) (3,246 ) (3,535 ) (3,391 ) Fair value of plan assets 15,330 15,489 19,401 6,030 6,322 5,136 2,498 2,564 1,897 1,178 (76 ) 60 (319 ) (468 ) (780 ) (748 ) (971 ) (1,494 ) Present value of unfunded obligations (74 ) (80 ) (75 ) (233 ) (260 ) (229 ) — — — Other post-employment liabilities — — — — — — (67 ) (78 ) (67 ) Net defined benefit asset/(liability) 1,104 (156 ) (15 ) (552 ) (728 ) (1,009 ) (815 ) (1,049 ) (1,561 ) Represented by: Liabilities (74 ) (536 ) (300 ) (783 ) (951 ) (1,134 ) (815 ) (1,049 ) (1,561 ) Assets 1,178 380 285 231 223 125 — — — 1,104 (156 ) (15 ) (552 ) (728 ) (1,009 ) (815 ) (1,049 ) (1,561 ) 1. Present value of funded obligations split approximately as follows: • UK pensions at 31 March 2018 : 10% active members ( 2017 : 12% ; 2016 : 12% ); 18% deferred members ( 2017 : 19% ; 2016 : 18% ); 72% pensioner members ( 2017 : 69% ; 2016 : 70% ) • US pensions at 31 March 2018 : 38% active members (2017: 38% ; 2016 : 39% ); 8% deferred members ( 2017 : 9% ; 2016 : 9% ); 54% pensioner members ( 2017 : 53% ; 2016 : 52% ) • US other post-retirement benefits at 31 March 2018 : 38% active members ( 2017 : 39% ; 2016 : 41% ); 0% deferred members ( 2017 : 0% ; 2016 : 0% ); 62% pensioner members ( 2017 : 61% ; 2016 : 59% ) The recognition of the pension assets in both the UK in relation to the National Grid UK Pension Scheme (NGUKPS) and the Niagara Mohawk Plan in the US reflect legal and actuarial advice that we have taken regarding recognition of surpluses under IFRIC 14. In both cases we have concluded that the Group has an unconditional right to a refund from the individual plans, including from each Section of the NGUKPS, in the event of a winding up. In the UK, the Trustees must seek the agreement of the Company to any benefit augmentation beyond the provisions set out in the Scheme Rules. In the US, the surplus assets may be used to pay benefits under other Plans, thereby allowing the Company to settle other liabilities under other Plans. 23. Pensions and other post-retirement benefits continued Amounts recognised in the income statement and statement of other comprehensive income 2018 2017 2016 £m £m £m Included within operating costs Administration costs 16 16 16 Included within payroll costs Defined benefit scheme costs: Current service cost 193 232 221 Past service cost – augmentations 1 1 3 Past service credit – redundancies (1 ) (1 ) (1 ) Special termination benefit cost – redundancies 9 7 11 202 239 234 Included within finance income and costs Net interest cost 65 105 112 Included within gain on disposal of discontinued operations Administration costs — 5 2 Disposal of UK Gas Distribution — 34 — — 39 2 Total included in income statement 1, 2 283 399 364 Remeasurement gains of pension assets and post-retirement benefit obligations 1,313 348 539 Exchange adjustments 175 (345 ) (81 ) Total included in the statement of other comprehensive income 2 1,488 3 458 1. Amounts recognised in the income statement include operating costs of £nil ( 2017 : £1 million ; 2016 : £1 million ); payroll costs of £nil ( 2017 : £35 million ; 2016 : £28 million ); and net interest of £nil ( 2017 : £2 million income; 2016 : £1 million cost) presented within profit from discontinued operations. These amounts all relate to UK pensions. 2. Amounts recognised in the statement of other comprehensive income include remeasurements of pension assets and post-retirement benefit obligations of £nil ( 2017 : £75 million loss; 2016 : £129 million gain) presented within discontinued operations. These amounts all relate to UK pensions. The geographical split of pensions and other post-retirement benefits is as shown below: UK Pensions US Pensions US other post-retirement benefits 2018 2017 2016 2018 2017 2016 2018 2017 2016 £m £m £m £m £m £m £m £m £m Included within operating costs Administration costs 6 6 9 9 9 6 1 1 1 Included within payroll costs Defined benefit scheme costs: Current service cost 49 76 74 98 103 95 46 53 52 Past service cost – augmentations 1 1 3 — — — — — — Past service credit – redundancies (1 ) (1 ) (1 ) — — — — — — Special termination benefit cost – redundancies 9 7 11 — — — — — — 58 83 87 98 103 95 46 53 52 Included within finance income and costs Net interest cost 3 — 18 27 43 36 35 62 58 Included within gain on disposal of discontinued operations Administration costs — 5 2 — — — — — — Disposal of UK Gas Distribution — 34 — — — — — — — — 39 2 — — — — — — Total included in income statement 67 128 116 134 155 137 82 116 111 Remeasurement gains/(losses) of pension assets and post retirement benefit obligations 1,177 (541 ) 534 27 319 (67 ) 109 570 72 Exchange adjustments — — — 75 (140 ) (33 ) 100 (205 ) (48 ) Total included in the statement of other comprehensive income 1,177 (541 ) 534 102 179 (100 ) 209 365 24 23. Pensions and other post-retirement benefits continued Reconciliation of the net defined benefit liability 2018 2017 2016 £m £m £m Opening net defined benefit liability (1,933 ) (2,585 ) (3,258 ) Cost recognised in the income statement (283 ) (399 ) (364 ) Remeasurement and foreign exchange effects recognised in the statement of other comprehensive income 1,488 3 458 Employer contributions 475 1,073 587 Other movements (10 ) (25 ) (8 ) Closing net defined benefit liability (263 ) (1,933 ) (2,585 ) The geographical split of pensions and other post-retirement benefits is as shown below: UK pensions US pensions US other post-retirement benefits 2018 2017 2016 2018 2017 2016 2018 2017 2016 £m £m £m £m £m £m £m £m £m Opening net defined benefit liability (156 ) (15 ) (672 ) (728 ) (1,009 ) (1,003 ) (1,049 ) (1,561 ) (1,583 ) Cost recognised in the income statement (67 ) (128 ) (116 ) (134 ) (155 ) (137 ) (82 ) (116 ) (111 ) Remeasurement and foreign exchange effects recognised in the statement of other comprehensive income 1,177 (541 ) 534 102 179 (100 ) 209 365 24 Employer contributions 150 528 239 208 257 231 117 288 117 Other movements — — — — — — (10 ) (25 ) (8 ) Closing net defined benefit 1,104 (156 ) (15 ) (552 ) (728 ) (1,009 ) (815 ) (1,049 ) (1,561 ) Changes in the present value of defined benefit obligations (including unfunded obligations) 2018 2017 2016 £m £m £m Opening defined benefit obligations (26,230 ) (28,952 ) (29,592 ) Current service cost (193 ) (232 ) (221 ) Interest cost (775 ) (1,057 ) (1,026 ) Actuarial (losses)/gains – experience (100 ) 166 659 Actuarial gains – demographic assumptions 671 225 — Actuarial gains/(losses) – financial assumptions 174 (3,377 ) 218 Past service credit – redundancies 1 1 1 Special termination benefit cost – redundancies (9 ) (7 ) (11 ) Past service cost – augmentations (1 ) (1 ) (3 ) Medicare subsidy received (21 ) (14 ) (15 ) Obligations transferred on disposal of UK Gas Distribution — 6,970 — Employee contributions (1 ) (1 ) (2 ) Benefits paid 1,285 1,443 1,348 Exchange adjustments 1,145 (1,394 ) (308 ) Closing defined benefit obligations (24,054 ) (26,230 ) (28,952 ) The geographical split of pensions and other post-retirement benefits is as shown below: UK pensions US pensions US other post-retirement benefits 2018 2017 2016 2018 2017 2016 2018 2017 2016 £m £m £m £m £m £m £m £m £m Opening defined benefit obligations (15,645 ) (19,416 ) (20,125 ) (7,050 ) (6,145 ) (6,055 ) (3,535 ) (3,391 ) (3,412 ) Current service cost (49 ) (76 ) (74 ) (98 ) (103 ) (95 ) (46 ) (53 ) (52 ) Interest cost (366 ) (615 ) (649 ) (273 ) (285 ) (242 ) (136 ) (157 ) (135 ) Actuarial (losses)/gains – experience (95 ) 106 552 (38 ) (2 ) 15 33 62 92 Actuarial gains – demographic assumptions 565 214 — 30 2 — 76 9 — Actuarial gains/(losses) – financial assumptions 604 (3,751 ) — (279 ) 37 120 (151 ) 337 98 Past service credit – redundancies 1 1 1 — — — — — — Special termination benefit cost – redundancies (9 ) (7 ) (11 ) — — — — — — Past service cost – augmentations (1 ) (1 ) (3 ) — — — — — — Medicare subsidy received — — — — — — (21 ) (14 ) (15 ) Obligations transferred on disposal of UK Gas Distribution — 6,970 — — — — — — — Employee contributions (1 ) (1 ) (2 ) — — — — — — Benefits paid 770 931 895 362 349 310 153 163 143 Exchange adjustments — — — 764 (903 ) (198 ) 381 (491 ) (110 ) Closing defined benefit obligations (14,226 ) (15,645 ) (19,416 ) (6,582 ) (7,050 ) (6,145 ) (3,246 ) (3,535 ) (3,391 ) 23. Pensions and other post-retirement benefits continued Changes in the value of plan assets 2018 2017 2016 £m £m £m Opening fair value of plan assets 24,375 26,434 26,408 Interest income 710 952 914 Return on plan assets in excess of/(less than) interest 568 3,334 (338 ) Administration costs (16 ) (21 ) (18 ) Employer contributions 475 1,073 587 Employee contributions 1 1 2 Benefits paid (1,285 ) (1,443 ) (1,348 ) Exchange adjustments (970 ) 1,049 227 Assets transferred on disposal of UK Gas Distribution — (7,004 ) — Closing fair value of plan assets 23,858 24,375 26,434 Actual return on plan assets 1,278 4,286 576 Expected contributions to plans in the following year 363 491 686 The geographical split of pensions and other post-retirement benefits is as shown below: UK pensions US pensions US other post-retirement benefits 2018 2017 2016 2018 2017 2016 2018 2017 2016 £m £m £m £m £m £m £m £m £m Opening fair value of plan assets 15,489 19,401 19,453 6,322 5,136 5,052 2,564 1,897 1,903 Interest income 363 615 631 246 242 206 101 95 77 Return on plan assets in excess 103 2,890 (18 ) 314 282 (202 ) 151 162 (118 ) Administration costs (6 ) (11 ) (11 ) (9 ) (9 ) (6 ) (1 ) (1 ) (1 ) Employer contributions 150 528 239 208 257 231 117 288 117 Employee contributions 1 1 2 — — — — — — Benefits paid (770 ) (931 ) (895 ) (362 ) (349 ) (310 ) (153 ) (163 ) (143 ) Exchange adjustments — — — (689 ) 763 165 (281 ) 286 62 Assets transferred on disposal of UK Gas Distribution — (7,004 ) — — — — — — — Closing fair value of plan assets 15,330 15,489 19,401 6,030 6,322 5,136 2,498 2,564 1,897 Actual return on plan assets 466 3,505 613 560 524 4 252 257 (41 ) Expected contributions to plans in the following year 140 128 228 221 229 220 2 134 238 Investment strategy Each plan’s investment strategy is formulated specifically in order to target specific asset allocations, asset returns and to manage risk. The asset allocation of the plans as at 31 March 2018 is as follows: UK pensions 1 US pensions 2 US other post-retirement benefits 3 % % % Equities 14.6 42.0 60.9 Corporate bonds 25.8 24.8 1.0 Government securities 36.7 16.2 20.4 Property 6.3 4.6 — Diversified alternatives 5.1 10.3 12.2 Liability matching assets 7.7 — — Other 3.8 2.1 5.5 100.0 100.0 100.0 1. Of total assets at year-end date, 57.8% is invested in the UK, 18.4% in the US and 23.7% in other countries, including the EU. 2. Of total assets at year-end date, 88.5% is invested in the US, 1.2% in the UK and 10.3% in other countries. 3. Of total assets at year-end date, 91.8% is invested within the US, 0.2% in the UK and 8.0% in other countries. 23. Pensions and other post-retirement benefits continued Investment strategy continued In the UK, the Trustees as governing body of each scheme are solely responsible for setting the investment strategy and managing risk. The Trustees' responsibilities are set out in the Trust Deeds and Rules. In the US, the fiduciary committee for each respective retirement plan is the Retirement Plan Committee (RPC). The RPC is solely responsible for setting the investment strategy and managing the risk of each plan. The RPC is structured in accordance with US laws governing retirement plans under the Employee Retirement Income Security Act (ERISA). The Trustees or the RPC where applicable, will perform the following duties: • After taking advice from professional investment advisers and consultation with the sponsoring employers, set the key principles, including expected returns, risk and liquidity requirements, after taking into account expected contributions, maturity of the pension liabilities and in the UK, the strength of the covenant. • Formulate an investment strategy to manage risk through diversification, including the use of liability matching assets, which move in line with long-term liabilities of the scheme, return seeking assets, some of which are designed to mitigate downside risk and exposure. Where appropriate, the strategies may include interest rate and inflation hedging instruments, and currency hedging to hedge foreign holdings. • Review the investment strategies’ performance and risk regularly and amend the portfolios if required. • Appoint investment managers with the necessary skills, expertise and credit worthiness to manage the investments. On a regular basis, the performance of the asset managers and their adherence to the investment strategies will be reviewed. Investments are made predominantly in regulated markets and investments outside of these markets are kept to prudent levels and subject to agreed control ranges. Asset allocations Within the asset allocations below there is significant diversification across regions, asset managers, currencies and bond categories. UK pensions 2018 2017 2016 Quoted Unquoted Total Quoted Unquoted Total Quoted Unquoted Total £m £m £m £m £m £m £m £m £m Equities 1 1,420 813 2,233 2,624 596 3,220 3,272 962 4,234 Corporate bonds 2 3,949 — 3,949 3,526 — 3,526 5,601 — 5,601 Government securities 5,629 — 5,629 5,406 — 5,406 6,059 — 6,059 Property 129 834 963 90 708 798 90 1,081 1,171 Diversified alternatives 3 99 690 789 250 628 878 159 505 664 Liability matching assets 4 1,174 — 1,174 1,162 — 1,162 1,020 — 1,020 Other 5 211 382 593 63 436 499 649 3 652 12,611 2,719 15,330 13,121 2,368 15,489 16,850 2,551 19,401 1. Included within equities at 31 March 2018 were ordinary shares of National Grid plc with a value of £nil ( 2017 : £2 million ; 2016 : £7 million ). 2. Included within corporate bonds at 31 March 2018 was an investment in a number of bonds issued by subsidiary undertakings with a value of £nil ( 2017 : £nil ; 2016 : £70 million ). 3. Includes return seeking non-conventional asset classes which include hedge funds, private debt, mezzanine debt, infrastructure debt and fixed income debt instruments. 4. Includes liability-driven investment vehicles. 5. Includes cash and cash type instruments. US pensions 2018 2017 2016 Quoted Unquoted Total Quoted Unquoted Total Quoted Unquoted Total £m £m £m £m £m £m £m £m £m Equities 577 1,954 2,531 698 1,915 2,613 625 1,508 2,133 Corporate bonds 1,085 413 1,498 1,130 537 1,667 954 483 1,437 Government securities 414 565 979 872 71 943 711 — 711 Property — 279 279 — 335 335 — 276 276 Diversified alternatives 1 198 421 619 209 433 642 163 334 497 Other 2 20 104 124 31 91 122 — 82 82 2,294 3,736 6,030 2,940 3,382 6,322 2,453 2,683 5,136 1. Includes return seeking non-conventional asset classes which include hedge funds, real estate debt and limited partnerships. 2. Includes collateral, liquid investments and cash deposits, debtors and creditors. US other post-retirement benefits 2018 2017 2016 Quoted Unquoted Total Quoted Unquoted Total Quoted Unquoted Total £m £m £m £m £m £m £m £m £m Equities 412 1,110 1,522 405 1,162 1,567 281 853 1,134 Corporate bonds 24 — 24 19 — 19 37 1 38 Government securities 508 2 510 520 1 521 390 — 390 Diversified alternatives 1 161 144 305 166 149 315 122 104 226 Other

Provisions

Provisions	12 Months Ended
	Mar. 31, 2018
Other Provisions, Contingent Liabilities And Contingent Assets [Abstract]
Provisions	Provisions We make provisions when an obligation exists, resulting from a past event and it is probable that cash will be paid to settle it, but the exact amount of cash required can only be estimated. The main estimates relate to environmental remediation and decommissioning costs for various sites we own or have owned and other provisions, including restructuring plans and lease contracts we have entered into that are now loss making. The evaluation of the likelihood of the contingent events has required best judgement by management regarding the probability of exposure to potential loss. Should circumstances change following unforeseeable developments, the likelihood could alter. Our strategy in action We are committed to the protection and enhancement of the environment. However, we have acquired, owned and operated a number of businesses which have, during the course of their operations, created an environmental impact. Therefore we have a provision that reflects the expected cost to remediate these sites. Current operations will seldom result in new sites with significant expected costs being added to the provision. Provisions are recognised where a legal or constructive obligation exists at the reporting date, as a result of a past event, where the amount of the obligation can be reliably estimated and where the outflow of economic benefit is probable. Provision is made for decommissioning and environmental costs, based on future estimated expenditures, discounted to present values. An initial estimate of decommissioning and environmental costs attributable to property, plant and equipment is recorded as part of the original cost of the related property, plant and equipment. Changes in the provision arising from revised estimates or discount rates or changes in the expected timing of expenditures that relate to property, plant and equipment are recorded as adjustments to their carrying value and depreciated prospectively over their remaining estimated useful economic lives; otherwise such changes are recognised in the income statement. The unwinding of the discount is included within the income statement as finance costs. Environmental £m Decommissioning £m Restructuring £m Emissions £m Other £m Total provisions £m At 1 April 2016 1,169 141 30 18 361 1,719 Exchange adjustments 137 13 — 2 36 188 Additions 572 107 16 12 308 1,015 Unused amounts reversed (75 ) — (5 ) — (6 ) (86 ) Unwinding of discount 60 — 1 — 12 73 Utilised (110 ) (19 ) (25 ) — (73 ) (227 ) Disposal of UK Gas Distribution (32 ) (21 ) — — (41 ) (94 ) At 31 March 2017 1,721 221 17 32 597 2,588 Exchange adjustments (158 ) (9 ) — (2 ) (26 ) (195 ) Additions 27 2 10 12 23 74 Unused amounts reversed 1 (45 ) (19 ) (6 ) — (37 ) (107 ) Unwinding of discount 61 5 1 — 8 75 Utilised 2 (75 ) (6 ) (7 ) (34 ) (146 ) (268 ) Transfers 3 — — (12 ) — (103 ) (115 ) At 31 March 2018 1,531 194 3 8 316 2,052 2018 2017 £m £m Current 273 416 Non-current 1,779 2,172 2,052 2,588 1. Unused amounts reversed from other provisions include £16 million in relation to discontinued operations. 2. Utilised amounts for other provisions include £77 million in relation to discontinued operations. 3. Represents net amounts transferred to trade and other payables (see note 21) of £115 million ( 2017 : £nil ). 24. Provisions continued Environmental provision The environmental provision represents the estimated restoration and remediation costs relating to a number of sites owned and managed by subsidiary undertakings, together with certain US sites that National Grid no longer owns. The environmental provision is as follows: 2018 2017 Discounted £m Undiscounted £m Real discount rate Discounted £m Undiscounted £m Real discount rate UK sites 213 235 1 % 242 270 1 % US sites 1,318 1,410 1 % 1,479 1,619 1 % 1,531 1,645 1,721 1,889 The remediation expenditure in the UK relates to old gas manufacturing sites and also to electricity transmission sites. Cash flows are expected to be incurred until 2075 although the weighted average duration of the cash flows is 11 years. A number of estimation uncertainties affect the calculation of the provision, including the impact of regulation, accuracy of the site surveys, unexpected contaminants, transportation costs, the impact of alternative technologies and changes in the real discount rate. This provision incorporates our best estimate of the financial effect of these uncertainties, but future changes in any of the assumptions could materially impact the calculation of the provision. The undiscounted amount is the undiscounted best estimate of the liability having regard to these uncertainties. The remediation expenditure in the US is expected to be incurred until 2069 . The weighted average duration of the cash flows is nine years. The increase in the US provision last year principally related to the reduction in real discount rate and expected expenditure in relation to three industrial sites located on or near waterways in New York that are subject to both state and federal law in the US. The uncertainties regarding the calculation of this provision are similar to those considered in respect of UK sites. This expenditure is expected to be largely recoverable from ratepayers under the terms of various rate agreements in the US. Decommissioning provision The decommissioning provision represents £71 million ( 2017 : £78 million ) of expenditure relating to asset retirement obligations estimated to be incurred until 2115 , and £104 million ( 2017 : £107 million ) of expenditure relating to the demolition of gas holders estimated to be incurred until 2023 . It also includes the net present value of the estimated expenditure (discounted at a real rate of 1% ) expected to be incurred until 2033 in respect of the decommissioning of certain US nuclear generating units that National Grid no longer owns. A pre-existing decommissioning provision formed part of the net assets sold as part of the UK Gas Distribution transaction. Other provisions Included within other provisions at 31 March 2018 are the following amounts in respect of the disposal of UK Gas Distribution: • £50 million ( 2017 : £143 million ) in respect of business restructuring costs following the sale of UK Gas Distribution. We expect the majority of this provision to be utilised within one year; and • Last year, other provisions included £150 million voluntary distribution to be made for the benefit of energy customers on the successful sale of UK Gas Distribution. During the year, this was transferred to trade and other payables and has a closing balance of £117 million . Other provisions also include: • Amounts provided in respect of onerous lease commitments and rates payable on surplus properties of £48 million ( 2017 : £84 million ) with expenditure expected to be incurred until 2043 ; • £152 million ( 2017 : £166 million ) of estimated liabilities in respect of past events insured by insurance subsidiary undertakings, including employer liability claims. In accordance with insurance industry practice, these estimates are based on experience from previous years and there is, therefore, no identifiable payment date; and • £13 million ( 2017 : £13 million ) in respect of obligations associated with investments in joint ventures and associates.

Share capital

Share capital	12 Months Ended
	Mar. 31, 2018
Share Capital, Reserves And Other Equity Interest [Abstract]
Share capital	Share capital Ordinary share capital represents the total number of shares issued which are publicly traded. We also disclose the number of treasury shares the Company holds, which are shares that the Company has bought itself, predominately to actively manage scrip issuances and employee share option plan liabilities. Share capital is accounted for as an equity instrument. An equity instrument is any contract that includes a residual interest in the consolidated assets of the Company after deducting all its liabilities and is recorded at the proceeds received, net of direct issue costs, with an amount equal to the nominal amount of the shares issued included in the share capital account and the balance recorded in the share premium account. Allotted, called up and fully paid million £m At 1 April 2016 3,924 447 Issued during the year in lieu of dividends 1 19 2 At 31 March 2017 3,943 449 Effect of share consolidation 2 (328 ) — Issued during the year in lieu of dividends 1 23 3 At 31 March 2018 3,638 452 1. The issue of shares under the scrip dividend programme is considered to be a bonus issue under the terms of the Companies Act 2006 and the nominal value of the shares is charged to the share premium account. 2. On 22 May 2017 the ordinary share capital was consolidated with 11 new ordinary shares of 12 204 / 473 pence nominal value issued for every 12 existing ordinary shares of 11 17 / 43 pence nominal value. This consolidation was completed to maintain the comparability of the Company’s share price before and after the special dividend. The share capital of the Company consists of ordinary shares of 12 204 ⁄ 473 pence nominal value each including ADSs. The ordinary shares and ADSs allow holders to receive dividends and vote at general meetings of the Company. The Company holds treasury shares but may not exercise any rights over these shares including the entitlement to vote or receive dividends. There are no restrictions on the transfer or sale of ordinary shares. In line with the provisions of the Companies Act 2006, the Company has amended its Articles of Association and ceased to have authorised share capital. Treasury shares At 31 March 2018 , the Company held 283 million ( 2017 : 193 million ) of its own shares. The market value of these shares as at 31 March 2018 was £2,270 million ( 2017 : £1,958 million ). The Company made the following transactions in respect of its own shares during the year ended 31 March 2018 : 1. During the year, the Company completed a share repurchase programme as part of the return of cash to shareholders following the sale of UK Gas Distribution in addition to the management of the dilutive effect of share issuances under the scrip dividend programme. As a result the Company repurchased 114 million ( 2017 : 20 million ) ordinary shares for aggregate consideration of £1,017 million ( 2017 : £189 million ), including transaction costs. The shares repurchased had a nominal value of £14 million ( 2017 : £2 million ) and represented approximately 3.1% ( 2017 : 0.5% ) of the ordinary shares in issue as at 31 March 2018 . 2. During the year, 3 million ( 2017 : 3 million ) treasury shares were gifted to National Grid Employee Share Trusts and 5 million ( 2017 : 3 million ) treasury shares were re-issued in relation to employee share schemes, in total representing approximately 0.2% ( 2017 : 0.2% ) of the ordinary shares in issue as at 31 March 2018 . The nominal value of these shares was £1 million ( 2017 : £1 million ) and the total proceeds received were £33 million ( 2017 : £18 million ). 3. During the year, the Company made payments totalling £5 million ( 2017 : £6 million ) to National Grid Employee Share Trusts, outside its share repurchase programme, to enable the trustees to make purchases of National Grid plc shares in order to satisfy the requirements of employee share option and reward plans. The maximum number of ordinary shares held in treasury during the year was 283 million ( 2017 : 194 million ) representing approximately 7.8% ( 2017 : 4.9% ) of the ordinary shares in issue as at 31 March 2018 and having a nominal value of £35 million ( 2017 : £22 million ).

Other equity reserves

Other equity reserves	12 Months Ended
	Mar. 31, 2018
Disclosure of reserves within equity [abstract]
Other equity reserves	Other equity reserves Other equity reserves are different categories of equity as required by accounting standards and represent the impact of a number of our historical transactions. Other equity reserves comprise the translation reserve (see accounting policy D in note 1), cash flow hedge reserve (see note 16), available-for-sale reserve (see note 14), the capital redemption reserve and the merger reserve. The merger reserve arose as a result of the application of merger accounting principles under the then prevailing UK GAAP, which under IFRS 1 was retained for mergers that occurred prior to the IFRS transition date. Under merger accounting principles, the difference between the carrying amount of the capital structure of the acquiring vehicle and that of the acquired business was treated as a merger difference and included within reserves. As the amounts included in other equity reserves are not attributable to any of the other classes of equity presented, they have been disclosed as a separate classification of equity. Translation £m Cash flow hedge £m Available- for-sale £m Capital redemption £m Merger £m Total £m At 1 April 2015 479 (109 ) 94 19 (5,165 ) (4,682 ) Exchange adjustments 1 69 — — — — 69 Net gains taken to equity — 50 43 — — 93 Transferred to profit or loss — 29 — — — 29 Tax — (15 ) (17 ) — — (32 ) At 31 March 2016 548 (45 ) 120 19 (5,165 ) (4,523 ) Exchange adjustments 1 346 — — — — 346 Net (losses)/gains taken to equity — (36 ) 81 — — 45 Transferred to/(from) profit or loss — 227 (25 ) — — 202 Tax — (43 ) (14 ) — — (57 ) At 31 March 2017 894 103 162 19 (5,165 ) (3,987 ) Exchange adjustments 1 (504 ) — — — — (504 ) Net gains/(losses) taken to equity — 19 (30 ) — — (11 ) Share of net gains taken to equity of associates — 5 — — — 5 Transferred from profit or loss — (3 ) (73 ) — — (76 ) Tax — 4 29 — — 33 At 31 March 2018 390 128 88 19 (5,165 ) (4,540 ) 1. The exchange adjustments recorded in the translation reserve comprise a loss of £1,304 million ( 2017 : gain of £1,364 million ; 2016 : gain of £275 million ) relating to the translation of foreign operations offset by a gain of £800 million ( 2017 : loss of £1,018 million ; 2016 : loss of £206 million ) relating to borrowings, cross-currency swaps and foreign exchange forward contracts used to hedge the net investment in the non-sterling denominated subsidiaries. The merger reserve represents the difference between the carrying value of subsidiary undertaking investments and their respective capital structures following the Lattice demerger from BG Group plc and the 1999 Lattice refinancing. The cash flow hedge reserve will amortise as the committed future cash flows from borrowings are paid or capitalised in fixed assets (as described in note 16). The amount due to be released from reserves next year is a gain of £23 million (pre-tax) of which £20 million relates to capital expenditure. The majority of the remaining reserve will be released over the next four years as forecast capital expenditure is incurred and over a similar maturity profile as the hedged borrowings.

Net debt

Net debt	12 Months Ended
	Mar. 31, 2018
Subclassifications of assets, liabilities and equities [abstract]
Net Debt	Net debt Net debt represents the amount of borrowings and overdrafts less cash, financial investments and related financing derivatives. Funding and liquidity risk management is carried out by the treasury function under policies and guidelines approved by the Finance Committee of the Board. The Finance Committee is responsible for the regular review and monitoring of treasury activity and for the approval of specific transactions, the authority for which fall outside the delegation of authority to management. The primary objective of the treasury function is to manage our funding and liquidity requirements. A secondary objective is to manage the associated financial risks, in the form of interest rate risk and foreign exchange risk, to within pre-authorised parameters. Details of the main risks arising from our financing and commodity hedging activities can be found in the risk factors discussion starting on page 193 and in note 30 to the consolidated financial statements on pages 158 to 164. Investment of surplus funds, usually in short-term fixed deposits or placements with money market funds that invest in highly liquid instruments of high credit quality, is subject to our counterparty risk management policy. (a) Reconciliation of net cash flow to movement in net debt 2018 2017 2016 £m £m £m (Decrease)/increase in cash and cash equivalents (807 ) 984 4 (Decrease)/increase in financial investments (5,953 ) 5,675 391 Decrease/(increase) in borrowings and related derivatives 1 1,209 (3,715 ) (1,100 ) Net interest paid on the components of net debt 2 808 1,955 810 Change in debt resulting from cash flows (4,743 ) 4,899 105 Changes in fair value of financial assets and liabilities and exchange movements 2,098 (2,273 ) (515 ) Net interest charge on the components of net debt (1,017 ) (2,401 ) (913 ) Disposal of UK Gas Distribution — 5,890 — Other non-cash movements (66 ) (64 ) (87 ) Movement in net debt (net of related derivative financial instruments) in the year (3,728 ) 6,051 (1,410 ) Net debt (net of related derivative financial instruments) at start of year (19,274 ) (25,325 ) (23,915 ) Net debt (net of related derivative financial instruments) at end of year (23,002 ) (19,274 ) (25,325 ) Composition of net debt Net debt is made up as follows: 2018 2017 2016 £m £m £m Cash, cash equivalents and financial investments 3,023 9,880 3,125 Borrowings and bank overdrafts (26,625 ) (28,638 ) (28,344 ) Financing derivatives 1 600 (516 ) (106 ) (23,002 ) (19,274 ) (25,325 ) 1. The financing derivatives balance included in net debt excludes the commodity derivatives and the Further Acquisition Agreement (FAA) derivative (see note 16). Included within the financing derivatives balance is £12 million ( 2017 : £18 million ; 2016 : £(16) million ) of derivative cash flows in relation to capital expenditure. This is included within investing activities and not financing activities in the cash flow statement. 2. Excludes £27 million ( 2017 : £nil ; 2016 : £nil ) cash interest from the Quadgas HoldCo Limited shareholder loan included within interest received in the cash flow statement. 27. Net debt continued (b) Analysis of changes in net debt Cash and cash equivalents £m Bank overdrafts £m Net cash and cash equivalents £m Financial investments £m Borrowings £m Financing derivatives £m Total 1 £m At 1 April 2016 127 (3 ) 124 2,998 (28,341 ) (106 ) (25,325 ) Cash flow 1,001 (17 ) 984 5,624 (2,196 ) 487 4,899 Fair value gains and losses and exchange movements 16 — 16 141 (1,527 ) (903 ) (2,273 ) Interest income/(charges) 2 — — — 53 (2,221 ) (233 ) (2,401 ) Other non-cash movements — — — — (294 ) 230 (64 ) Disposal (5 ) 20 15 (75 ) 5,941 9 5,890 At 31 March 2017 1,139 — 1,139 8,741 (28,638 ) (516 ) (19,274 ) Cash flow 2 (807 ) — (807 ) (5,983 ) 2,108 (61 ) (4,743 ) Fair value gains and losses and exchange movements (3 ) — (3 ) (149 ) 1,088 1,162 2,098 Interest income/(charges) 2 — — — 85 (1,117 ) 15 (1,017 ) Other non-cash movements — — — — (66 ) — (66 ) At 31 March 2018 329 — 329 2,694 (26,625 ) 600 (23,002 ) Balances at 31 March 2018 comprise: Non-current assets — — — — — 1,170 1,170 Current assets 329 — 329 2,694 — 375 3,398 Current liabilities — — — — (4,447 ) (620 ) (5,067 ) Non-current liabilities — — — — (22,178 ) (325 ) (22,503 ) 329 — 329 2,694 (26,625 ) 600 (23,002 ) 1. Includes accrued interest at 31 March 2018 of £197 million ( 2017 : £210 million ; 2016 : £243 million ). 2. An exceptional income of £3 million ( 2017 : £1,313 million expense; 2016 : £nil ) is included in net interest charge on the components of net debt and an exceptional cash outflow of £231 million ( 2017 : £1,052 million ; 2016 : £nil ) is included in cash flow on the components of net debt.

Commitments and contingencies

Commitments and contingencies	12 Months Ended
	Mar. 31, 2018
Other Provisions, Contingent Liabilities And Contingent Assets [Abstract]
Commitments and contingencies	Commitments and contingencies Commitments are those amounts that we are contractually required to pay in the future as long as the other party meets its obligations. These commitments primarily relate to operating lease rentals, energy purchase agreements and contracts for the purchase of assets which, in many cases, extend over a long period of time. We also disclose any contingencies, which include guarantees that companies have given, where we pledge assets against current obligations that will remain for a specific period. 2018 2017 £m £m Future capital expenditure Contracted for but not provided 1 1,843 1,913 Operating lease commitments Less than 1 year 68 95 In 1 to 2 years 44 82 In 2 to 3 years 40 58 In 3 to 4 years 37 56 In 4 to 5 years 35 54 More than 5 years 219 274 443 619 Energy purchase commitments 2 Less than 1 year 1,237 1,325 In 1 to 2 years 700 744 In 2 to 3 years 563 587 In 3 to 4 years 449 507 In 4 to 5 years 410 436 More than 5 years 1,969 2,100 5,328 5,699 Guarantees Guarantee of sublease for US property (expires 2040) 178 225 Guarantees of certain obligations of Grain LNG Import Terminal (expire up to 2028) 46 100 Guarantees of certain obligations for construction of HVDC West Coast Link (expires 2018) 213 281 Guarantees of certain obligations of Nemo Link Limited (various expiry dates) 63 140 Guarantees of certain obligations of National Grid North Sea Link Limited (various expiry dates) 3 1,009 1,059 Guarantees of certain obligations of St William Homes LLP (various expiry dates) 4 98 147 Guarantees of certain obligations for construction of IFA 2 SAS (expected expiry 2021) 3 729 354 Other guarantees and letters of credit (various expiry dates) 333 474 2,669 2,780 1. Following a review in the year, the basis on which we disclose capital commitments has been refined. 2. Energy purchase commitments relate to contractual commitments to purchase electricity or gas that are used to satisfy physical delivery requirements to our customers or for energy that we use ourselves (i.e. normal purchase, sale or usage) and hence are accounted for as ordinary purchase contracts. Details of commodity contract derivatives that do not meet the normal purchase, sale or usage criteria, and hence are accounted for as derivative contracts, are shown in note 16(b). 3. Included within total guarantees are guarantees to both joint ventures and EPC contractors regarding the construction of interconnectors of £739 million (2017: £555 million ). 4. Includes guarantees to related parties. The total of future minimum sublease payments expected to be received under non-cancellable subleases is £42 million ( 2017 : £15 million ). Through the ordinary course of our operations, we are party to various litigation, claims and investigations. We do not expect the ultimate resolution of any of these proceedings to have a material adverse effect on our results of operations, cash flows or financial position.

Related party transactions

Related party transactions	12 Months Ended
	Mar. 31, 2018
Related party transactions [abstract]
Related party transactions	Related party transactions Related parties include joint ventures, associates, investments and key management personnel. The following significant transactions with related parties were in the normal course of business. Amounts receivable from and payable to related parties are due on normal commercial terms: 2018 2017 2016 £m £m £m Sales: Goods and services supplied to a pension plan 3 3 3 Sales: Goods and services supplied to joint ventures 1 14 78 9 Sales: Goods and services supplied to associates 2 220 — 4 Purchases: Goods and services received from joint ventures 3 135 168 183 Purchases: Goods and services received from associates 3 160 169 83 Receivable from joint ventures 4 160 64 7 Receivable from associates 4 376 457 — Payable to joint ventures 5 — 84 96 Payable to associates 17 27 7 Interest income from joint ventures 4 — — Interest income from associates 27 — — Dividends received from joint ventures 6 43 75 48 Dividends received from associates 7 170 24 24 1. In 2018 £5 million (2017: £68 million ) of property sites were sold to joint venture St William Homes LLP. 2. Sales in the year relate to transactions with Quadgas HoldCo Limited. Within this is other income of £54 million relating to a Transitional Service Agreement following the sale of the UK Gas Distribution business to Quadgas HoldCo Limited. 3. During the year the Group received goods and services from a number of US associates, both for the transportation of gas and for pipeline services in the US. Additionally, goods and services were received from UK joint ventures for the construction of a transmission link in the UK. 4. Amounts receivable from associates includes a loan receivable balance from Quadgas HoldCo Limited of £352 million (2017: £434 million ) and a loan receivable balance of £130 million (2017: £61 million ) from Nemo Link Limited (a joint venture). 5. In previous years the amounts payable to joint ventures include deposits received for National Grid property sites from St William Homes LLP which have been settled during the year. 6. Dividends in respect of joint ventures were received from BritNed Development Limited. 7. Within dividends received from associates in 2018, £144 million (2017: £nil ) was from Quadgas HoldCo Limited. Details of investments in principal subsidiary undertakings, joint ventures and associates are disclosed in note 32 and information relating to pension fund arrangements is disclosed in note 23. For details of Directors’ and key management remuneration, refer to the Remuneration Report and note 3(c). As a result of an overpayment due to a payroll processing error in September 2017, the Company was owed $70,767 by Dean Seavers (Executive Director, US and a Director of National Grid plc), which was repaid in full in early October 2017.

Financial risk management

Financial risk management	12 Months Ended
	Mar. 31, 2018
Financial Instruments [Abstract]
Financial risk management	Financial risk management Our activities expose us to a variety of financial risks including currency risk, interest rate risk, commodity price risk, credit risk, capital risk and liquidity risk. Our risk management programme focuses on the unpredictability of financial markets and seeks to minimise potential volatility of financial performance from these risks. We use financial instruments, including derivative financial instruments, to manage risks of this type. This note describes our approach to managing risk, including an analysis of assets and liabilities by currency type and an analysis of interest rate category for our net debt. We are required by accounting standards to also include a number of specific disclosures (such as a maturity analysis of contractual undiscounted cash flows) and have included these requirements below. Risk management related to financing activities is carried out by a central treasury department under policies approved by the Finance Committee of the Board. The objective of the treasury department is to manage funding and liquidity requirements, including managing associated financial risks, to within acceptable boundaries. The Finance Committee provides written principles for overall risk management, as well as written policies covering specific areas such as foreign exchange risk, interest rate risk, credit risk, liquidity risk, use of derivative financial instruments and non-derivative financial instruments, and investment of excess liquidity. We have exposure to the following risks, which are described in more detail below: • credit risk; • liquidity risk; • interest rate risk; • currency risk; and • capital risk. 30. Financial risk management continued (a) Credit risk We are exposed to the risk of loss resulting from counterparties’ default on their commitments including failure to pay or make a delivery on a contract. This risk is inherent in our commercial business activities. We are exposed to credit risk on our cash and cash equivalents, derivative financial instruments, deposits with banks and financial institutions, as well as credit exposures to wholesale and retail customers, including outstanding receivables and committed transactions. Treasury credit risk Counterparty risk arises from the investment of surplus funds and from the use of derivative financial instruments. As at 31 March 2018, the following limits were in place for investments held with banks and financial institutions: Maximum limit Long-term limit Triple ‘A’ G7 sovereign entities (AAA) 1,853 927 Triple ‘A’ vehicles (AAA) 500 — Triple ‘A’ range institutions and non G7 sovereign entities (AAA) 1,011 506 Double ‘A+’ G7 sovereign entities (AA+) 1,685 843 Double ‘A’ range institutions (AA) 674 to 843 337 to 421 Single ‘A’ range institutions (A) 236 to 337 118 to 169 The maximum limit applies to all transactions, including long-term transactions. The long-term limit applies to transactions which mature in more than 12 months ’ time. As at 31 March 2017 and 2018, we had a number of exposures to individual counterparties. In accordance with our treasury policies, counterparty credit exposure utilisations are monitored daily against the counterparty credit limits. Counterparty credit ratings and market conditions are reviewed continually with limits being revised and utilisation adjusted, if appropriate. Management does not expect any significant losses from non performance by these counterparties. Commodity credit risk The credit policy for US-based commodity transactions is owned by the Finance Committee to the Board, which establishes controls and procedures to determine, monitor and minimise the credit exposure to counterparties. Authority to administer the policy has been delegated to the Energy Procurement Risk Management Committee by the Board and the Executive Committee. Wholesale and retail credit risk Our principal commercial exposure in the UK is governed by the credit rules within the regulated codes: Uniform Network Code and Connection and Use of System Code. These set out the level of credit relative to the RAV for each credit rating. In the US, we are required to supply electricity and gas under state regulations. Our credit policies and practices are designed to limit credit exposure by collecting security deposits prior to providing utility services, or after utility service has commenced if certain applicable regulatory requirements are met. Collection activities are managed on a daily basis. Sales to retail customers are usually settled in cash, cheques, electronic bank payments or by using major credit cards. We are committed to measuring, monitoring, minimising and recording counterparty credit risk in our wholesale business. The utilisation of credit limits is regularly monitored and collateral is collected against these accounts when necessary. Management does not expect any significant losses of receivables that have not been provided for as shown in note 18. 30. Financial risk management continued (a) Credit risk continued Offsetting financial assets and liabilities The following tables set out our financial assets and liabilities which are subject to offset and to enforceable master netting arrangements or similar agreements. The tables show the amounts which are offset and reported net in the statement of financial position. Amounts which cannot be offset under IFRS, but which could be settled net under terms of master netting agreements if certain conditions arise, and with collateral received or pledged, are shown to present National Grid’s net exposure. Financial assets and liabilities on different transactions are only reported net if the transactions are with the same counterparty, a currently enforceable legal right of offset exists and the cash flows are intended to be settled on a net basis. Amounts which do not meet the criteria for offsetting on the statement of financial position, but could be settled net in certain circumstances, principally relate to derivative transactions under ISDA agreements where each party has the option to settle amounts on a net basis in the event of default of the other party. Commodity contract derivatives that have not been offset on the balance sheet may be settled net in certain circumstances under ISDA or NAESB (North American Energy Standards Board) agreements. National Grid has similar arrangements in relation to bank account balances and bank overdrafts, and trade payables and trade receivables which are subject to general terms and conditions. However, these balances are immaterial. Related amounts At 31 March 2018 Gross £m Gross amounts offset £m Net amount £m Financial instruments £m Cash £m Net amount £m Assets Financing derivatives 1,545 — 1,545 (523 ) (772 ) 250 Commodity contract derivatives 69 — 69 (16 ) — 53 Further Acquisition Agreement derivative 1 110 — 110 — — 110 1,724 — 1,724 (539 ) (772 ) 413 Liabilities Financing derivatives (945 ) — (945 ) 523 326 (96 ) Commodity contract derivatives (116 ) — (116 ) 16 7 (93 ) (1,061 ) — (1,061 ) 539 333 (189 ) 663 — 663 — (439 ) 224 1. The Group held a put/call option as at 31 March 2017. The fair value was £nil. Related amounts At 31 March 2017 Gross carrying amounts £m Gross amounts offset £m Net amount presented in statement of financial position £m Financial instruments £m Cash collateral received/ pledged £m Net amount £m Assets Financing derivatives 1,707 — 1,707 (718 ) (692 ) 297 Commodity contract derivatives 106 — 106 (25 ) — 81 1,813 — 1,813 (743 ) (692 ) 378 Liabilities Financing derivatives (2,223 ) — (2,223 ) 718 1,230 (275 ) Commodity contract derivatives (170 ) — (170 ) 25 18 (127 ) (2,393 ) — (2,393 ) 743 1,248 (402 ) (580 ) — (580 ) — 556 (24 ) 30. Financial risk management continued (b) Liquidity risk Our policy is to determine our liquidity requirements by the use of both short-term and long-term cash flow forecasts. These forecasts are supplemented by a financial headroom analysis which is used to assess funding requirements for at least a 24-month period and maintain adequate liquidity for a continuous 12 -month period. We believe our contractual obligations, including those shown in commitments and contingencies in note 28 can be met from existing cash and investments, operating cash flows and other financings that we reasonably expect to be able to secure in the future, together with the use of committed facilities if required. Our debt agreements and banking facilities contain covenants, including those relating to the periodic and timely provision of financial information by the issuing entity and financial covenants such as restrictions on the level of subsidiary indebtedness. Failure to comply with these covenants, or to obtain waivers of those requirements, could in some cases trigger a right, at the lender’s discretion, to require repayment of some of our debt and may restrict our ability to draw upon our facilities or access the capital markets. The following is an analysis of the contractual undiscounted cash flows payable under financial liabilities and derivative assets and liabilities as at the reporting date: At 31 March 2018 Less than £m 1 to 2 £m 2 to 3 £m More than £m Total £m Non-derivative financial liabilities Borrowings, excluding finance lease liabilities (4,099 ) (1,642 ) (2,325 ) (18,023 ) (26,089 ) Interest payments on borrowings 1 (730 ) (692 ) (629 ) (12,897 ) (14,948 ) Finance lease liabilities (60 ) (60 ) (45 ) (148 ) (313 ) Other non interest-bearing liabilities (2,840 ) (359 ) — — (3,199 ) Derivative financial liabilities Derivative contracts – receipts 1,069 601 130 250 2,050 Derivative contracts – payments (890 ) (263 ) (188 ) (529 ) (1,870 ) Commodity contract derivatives (80 ) (33 ) (26 ) 1 (138 ) (7,630 ) (2,448 ) (3,083 ) (31,346 ) (44,507 ) At 31 March 2017 Less than 1 year £m 1 to 2 years £m 2 to 3 years £m More than 3 years £m Total £m Non-derivative financial liabilities Borrowings, excluding finance lease liabilities (5,142 ) (1,864 ) (1,750 ) (19,245 ) (28,001 ) Interest payments on borrowings 1 (767 ) (707 ) (670 ) (12,975 ) (15,119 ) Finance lease liabilities (66 ) (58 ) (61 ) (183 ) (368 ) Other non interest-bearing liabilities (2,989 ) (260 ) — — (3,249 ) Derivative financial liabilities Derivative contracts – receipts 571 961 572 234 2,338 Derivative contracts – payments (1,551 ) (959 ) (304 ) (610 ) (3,424 ) Commodity contract derivatives (15 ) (18 ) (8 ) — (41 ) (9,959 ) (2,905 ) (2,221 ) (32,779 ) (47,864 ) 1. The interest on borrowings is calculated based on borrowings held at 31 March without taking account of future issues. Floating rate interest is estimated using a forward interest rate curve as at 31 March. Payments are included on the basis of the earliest date on which the Company can be required to settle. (c) Interest rate risk National Grid’s interest rate risk arises from our long-term borrowings. Borrowings issued at variable rates expose National Grid to cash flow interest rate risk, partially offset by cash held at variable rates. Borrowings issued at fixed rates expose National Grid to fair value interest rate risk. Our interest rate risk management policy is to seek to minimise total financing costs (being interest costs and changes in the market value of debt) subject to constraints. We do this by using fixed and floating rate debt and derivative financial instruments including interest rate swaps, swaptions and forward rate agreements. We hold some borrowings on issue that are inflation linked. We believe that these provide a partial economic offset to the inflation risk associated with our UK inflation linked revenues. The table in note 20 sets out the carrying amount, by contractual maturity, of borrowings that are exposed to interest rate risk before taking into account interest rate swaps. 30. Financial risk management continued (c) Interest rate risk continued During 2018 and 2017, net debt was managed using derivative financial instruments to hedge interest rate risk as follows: 2018 2017 Fixed rate £m Floating rate £m Inflation linked £m Other 1 £m Total £m Fixed rate £m Floating rate £m Inflation linked £m Other 1 £m Total £m Cash and cash equivalents — 302 — 27 329 940 199 — — 1,139 Financial investments 31 2,625 — 38 2,694 44 8,691 — 6 8,741 Borrowings (16,144 ) (3,191 ) (7,290 ) — (26,625 ) (17,681 ) (3,917 ) (7,040 ) — (28,638 ) Pre-derivative position (16,113 ) (264 ) (7,290 ) 65 (23,602 ) (16,697 ) 4,973 (7,040 ) 6 (18,758 ) Derivative effect 1,735 (1,237 ) 102 — 600 1,424 (1,785 ) (155 ) — (516 ) Net debt position (14,378 ) (1,501 ) (7,188 ) 65 (23,002 ) (15,273 ) 3,188 (7,195 ) 6 (19,274 ) 1. Represents financial instruments which are not directly affected by interest rate risk, such as investments in equity or other similar financial instruments. (d) Currency risk National Grid operates internationally and is exposed to foreign exchange risk arising from various currency exposures, primarily with respect to the US dollar. Foreign exchange risk arises from future commercial transactions, recognised assets and liabilities, and investments in foreign operations. Our policy for managing foreign exchange transaction risk is to hedge contractually committed foreign currency cash flows over a prescribed minimum size. Where foreign currency cash flow forecasts are less certain, our policy is to hedge a proportion of such cash flows. Instruments used to manage foreign exchange transaction risk include foreign exchange forward contracts and foreign exchange swaps. Our policy for managing foreign exchange translation risk relating to our net investment in foreign operations is to maintain cross-currency swaps and foreign exchange forwards so as to provide an economic offset. The primary managed foreign exchange exposure arises from the dollar denominated assets and liabilities held by our US operations, with a further small euro exposure in respect of joint venture investments. During 2018 and 2017, derivative financial instruments were used to manage foreign currency risk as follows: 2018 2017 Sterling £m Euro £m Dollar £m Other £m Total £m Sterling £m Euro £m Dollar £m Other £m Total £m Cash and cash equivalents 294 2 33 — 329 1,110 — 29 — 1,139 Financial investments 1,471 69 1,125 29 2,694 6,824 98 1,753 66 8,741 Borrowings (10,912 ) (3,794 ) (11,068 ) (851 ) (26,625 ) (11,099 ) (5,373 ) (10,729 ) (1,437 ) (28,638 ) Pre-derivative position (9,147 ) (3,723 ) (9,910 ) (822 ) (23,602 ) (3,165 ) (5,275 ) (8,947 ) (1,371 ) (18,758 ) Derivative effect 3,748 3,793 (7,992 ) 1,051 600 2,310 6,241 (10,708 ) 1,641 (516 ) Net debt position (5,399 ) 70 (17,902 ) 229 (23,002 ) (855 ) 966 (19,655 ) 270 (19,274 ) The exposure to dollars largely relates to our net investment hedge activities; exposure to euros largely relates to hedges for our future non-sterling capital expenditure as described in note 16. The currency exposure on other financial instruments is as follows: 2018 2017 Sterling £m Euro £m Dollar £m Other £m Total £m Sterling £m Euro £m Dollar £m Other £m Total £m Trade and other receivables 253 — 1,528 — 1,781 83 — 1,660 — 1,743 Trade and other payables (1,124 ) — (1,793 ) — (2,917 ) (1,209 ) — (1,795 ) — (3,004 ) Other non-current liabilities (144 ) — (254 ) — (398 ) (100 ) — (315 ) — (415 ) The carrying amounts of other financial instruments are denominated in the above currencies, which in most instances are the functional currency of the respective subsidiaries. Our exposure to dollars is due to activities in our US subsidiaries. We do not have any other significant exposure to currency risk on these balances. 30. Financial risk management continued (e) Capital risk management The capital structure of the Group consists of shareholders’ equity, as disclosed in the consolidated statement of changes in equity, and net debt (note 27). National Grid’s objectives when managing capital are: to safeguard our ability to continue as a going concern; to remain within regulatory constraints of our regulated operating companies; and to maintain an efficient mix of debt and equity funding thus achieving an optimal capital structure and cost of capital. We regularly review and manage the capital structure as appropriate in order to achieve these objectives. Maintaining appropriate credit ratings for our operating and holding companies is an important aspect of our capital risk management strategy and balance sheet efficiency. As noted on page 24, we monitor our balance sheet efficiency using several metrics including retained cash flow/net debt (RCF), interest cover and gearing. Retained cash flow/net debt and interest cover for the Group for the year ended 31 March 2018 were 9.7% ( 2017 : 14.9% ) and 4.4 ( 2017 : 5.0 ) respectively. We believe these are consistent with the current credit ratings for National Grid plc and the main operating companies within the Group, based on guidance from the rating agencies. We monitor the RAV gearing within each of NGET and the regulated transmission businesses within NGG. This is calculated as net debt expressed as a percentage of RAV, and indicates the level of debt employed to fund our UK regulated businesses. It is compared with the level of RAV gearing indicated by Ofgem as being appropriate for these businesses, at around 60 % to 62.5% . We also monitor net debt as a percentage of rate base for our US operating companies, comparing this with the allowed rate base gearing inherent within each of our agreed rate plans. The majority of our regulated operating companies in the US and the UK are subject to certain restrictions on the payment of dividends by administrative order, contract and/or licence. The types of restrictions that a company may have that would prevent a dividend being declared or paid unless they are met include: • dividends must be approved in advance by the relevant US state regulatory commission; • the subsidiary must have at least two recognised rating agency credit ratings of at least investment grade; • dividends must be limited to cumulative retained earnings, including pre-acquisition retained earnings; • National Grid plc must maintain an investment grade credit rating and if that rating is the lowest investment grade bond rating it cannot have a negative watch/review for downgrade notice by a credit rating agency; • the subsidiary must not carry on any activities other than those permitted by the licences; • the subsidiary must not create any cross-default obligations or give or receive any intra-group cross-subsidies; and • the percentage of equity compared with total capital of the subsidiary must remain above certain levels. There is a further restriction relating only to the Narragansett Electric Company, which is required to maintain its consolidated net worth above certain levels. These restrictions are subject to alteration in the US as and when a new rate case or rate plan is agreed with the relevant regulatory bodies for each operating company and in the UK through the normal licence review process. As most of our business is regulated, at 31 March 2018 the majority of our net assets are subject to some of the restrictions noted above. These restrictions are not considered to be significantly onerous, nor do we currently expect they will prevent the planned payment of dividends in future in line with our dividend policy. Some of our regulatory and bank loan agreements additionally impose lower limits for the long-term credit ratings that certain companies within the Group must hold. All the above requirements are monitored on a regular basis in order to ensure compliance. The Group has complied with all externally imposed capital requirements to which it is subject. (f) Fair value analysis Included in the statement of financial position are financial instruments which have been measured at fair value. These fair values can be categorised into hierarchy levels that are representative of the inputs used in measuring the fair value. The best evidence of fair value is a quoted price in an actively traded market. In the event that the market for a financial instrument is not active, a valuation technique is used. 2018 2017 Level 1 £m Level 2 £m Level 3 £m Total £m Level 1 £m Level 2 £m Level 3 £m Total £m Assets Investments – available-for-sale 2,406 310 5 2,721 7,717 315 5 8,037 Investments – joint ventures and associates — — 79 79 — — 41 41 Financing derivatives — 1,544 1 1,545 — 1,692 15 1,707 Commodity contract derivatives — 8 61 69 — 22 84 106 Further Acquisition Agreement derivative — — 110 110 — — — — 2,406 1,862 256 4,524 7,717 2,029 145 9,891 Liabilities Financing derivatives — (725 ) (220 ) (945 ) — (1,743 ) (480 ) (2,223 ) Commodity contract derivatives — (54 ) (62 ) (116 ) — (70 ) (100 ) (170 ) — (779 ) (282 ) (1,061 ) — (1,813 ) (580 ) (2,393 ) 2,406 1,083 (26 ) 3,463 7,717 216 (435 ) 7,498 Level 1: Financial instruments with quoted prices for identical instruments in active markets. Level 2: Financial instruments with quoted prices for similar instruments in active markets or quoted prices for identical or similar instruments in inactive markets, and financial instruments valued using models where all significant inputs are based directly or indirectly on observable market data. Level 3: Financial instruments valued using valuation techniques where one or more significant inputs are based on unobservable market data. 30. Financial risk management continued (f) Fair value analysis continued Our level 3 derivative financial instruments include cross-currency swaps, inflation linked swaps and equity options, all of which are traded on illiquid markets. In valuing these instruments we use in-house valuation models and obtain external valuations to support each reported fair value. Our level 3 commodity contract derivatives primarily consist of our forward purchases of electricity and gas where pricing inputs are unobservable, as well as other complex transactions. Complex transactions can introduce the need for internally developed models based on reasonable assumptions. Industry standard valuation techniques such as the Black-Scholes pricing model and Monte Carlo simulation are used for valuing such instruments. Level 3 is also applied in cases when optionality is present or where an extrapolated forward curve is considered unobservable. We consider forward curves to be unobservable if observed market transactions differ from the curve by more than 5% . Our level 3 investments include investments in associates relating to Sunrun Neptune Investor 2016 LLC, accounted at fair value (see note 15), and £5 million Series B preferred stocks in Enbala Holdings, Inc., accounted for as an available-for-sale investment. The Group is also party to the Further Acquisition Agreement (FAA) which contains a put/call option over 14% of the loan and equity it holds in Cadent (through its investment in Quadgas HoldCo Limited). The FAA is a derivative, which is accounted for at fair value, and the assumptions which are used to determine fair value are specific to the contract and not readily observable in active markets. The fair value of the option is £110 million ( 2017 : £ nil ). The changes in value of our level 3 derivative financial instruments are as follows: Financing derivatives/ Further Acquisition Agreement derivative Commodity contract derivatives Investments in associates and available-for-sale investments Total 2018 2017 2018 2017 2018 2017 2018 2017 £m £m £m £m £m £m £m £m At 1 April (465 ) (196 ) (16 ) (27 ) 46 — (435 ) (223 ) Net gains/(losses) for the year 1,2 125 (35 ) 8 (2 ) — — 133 (37 ) Purchases — — 27 15 41 46 68 61 Settlements 231 — (20 ) (2 ) (3 ) — 208 (2 ) Reclassification into level 3 — (234 ) — — — — — (234 ) At 31 March (109 ) (465 ) (1 ) (16 ) 84 46 (26 ) (435 ) 1. Gain of £125 million ( 2017 : £35 million loss) is attributable to derivative financial instruments held at the end of the reporting period and has been recognised in finance costs in the income statement. 2. Gain of £35 million ( 2017 : £21 million loss) is attributable to commodity contract derivative financial instruments held at the end of the reporting period. The impacts on a post-tax basis of reasonably possible changes in significant level 3 assumptions are as follows: Financing derivatives Commodity contract derivatives 2018 2017 2018 2017 £m £m £m £m 10% increase in commodity prices 1 — — (1 ) 1 10% decrease in commodity prices 1 — — 3 — Volume forecast uplift 2 — — — (1 ) Volume forecast reduction 2 — — — 1 +10% market area price change — — (6 ) (13 ) -10% market area price change — — 5 9 +20 basis points change in Limited Price Inflation (LPI) market curve 3 (84 ) (93 ) — — -20 basis points change in LPI market curve 3 82 88 — — 1. Level 3 commodity price sensitivity is included within the sensitivity analysis disclosed in note 33. 2. Volumes were flexed using maximum and minimum historical averages, or by > 10% where historical averages were not available. 3. A reasonably possible change in assumption of other level 3 derivative financial instruments is unlikely to result in a material change in fair values. The impacts disclosed above were considered on a contract by contract basis with the most significant unobservable inputs identified. The level 3 investments were acquired in the period on market terms and sensitivity is considered insignificant at 31 March 2018 . A 50 basis point change in the discount rate is used to determine the sensitivity of the fair value of our investment in Sunrun Neptune 2016 LLC. A 50 basis point increase/(decrease) would (decrease)/increase the fair value by £(5) million / £6 million (2017: £(8) million / £8 million ) respectively. A 5 percentage point decrease in the offer price received from Quadgas Investments BidCo Limited on 30 April 2018 applied in valuing the FAA would increase the fair value by approximately £39 million . Sensitivities In order to give a clearer picture of the impact on our results or financial position of potential changes in significant estimates and assumptions, the following sensitivities are presented. These sensitivities are hypothetical, as they are based on assumptions and conditions prevailing at the year-end, and should be used with caution. The effects provided are not necessarily indicative of the actual effects that would be experienced because our actual exposures are constantly changing. The sensitivities in the tables below show the potential impact in the income statement (and consequential impact on net assets) for a reasonably possible range of different variables each of which have been considered in isolation (i.e. with all other variables remaining constant). There are a number of these sensitivities which are mutually exclusive and therefore if one were to happen, another would not, meaning a total showing how sensitive our results are to these external factors is not meaningful. The sensitivities included in the tables below broadly have an equal and opposite effect if the sensitivity increases or decreases by the same amount unless otherwise stated. (a) Sensitivities on areas of estimation uncertainty The table below sets out the sensitivity analysis for each of the areas of estimation uncertainty set out in note 1E. These estimates are those that have a significant risk of resulting in a material adjustment to the carrying values of assets and liabilities in the next year. 2018 2017 Income statement £m Net assets £m Income statement £m Net assets £m Pensions and other post-retirement benefits 1 (pre-tax): UK discount rate change of 0.5% 2 8 1,075 9 1,305 US discount rate change of 0.5% 2 15 623 17 669 UK RPI rate change of 0.5% 3 5 965 8 1,114 UK long-term rate of increase in salaries change of 0.5% 4 — 61 2 80 US long-term rate of increase in salaries change of 0.5% 3 44 3 51 UK change of one year to life expectancy at age 65 2 588 2 673 US change of one year to life expectancy at age 65 4 359 4 365 Assumed US healthcare cost trend rates change of 1% 31 448 37 510 Environmental provision: 10% change in estimated future cash flows 154 154 175 175 0.5% change in discount rate 56 56 67 67 1. The changes shown are a change in the annual pension or other post-retirement benefit service charge and change in the defined benefit obligations. 2. A change in the discount rate is likely to occur as a result of changes in bond yields and as such would be expected to be offset to a significant degree by a change in the value of the bond assets held by the plans. 3. The projected impact resulting from a change in RPI reflects the underlying effect on pensions in payment, pensions in deferment and resultant increases in salary assumptions. 4. This change has been applied to both the pre 1 April 2014 and post 1 April 2014 rate of increase in salary assumption. Pensions and other post-retirement benefits assumptions Sensitivities have been prepared to show how the defined benefit obligations and annual service costs could potentially be impacted by changes in the relevant actuarial assumption that were reasonably possible as at 31 March 2018. In preparing sensitivities the potential impact has been calculated by applying the change to each assumption in isolation and assuming all other assumptions remain unchanged. This is with the exception of RPI in the UK where the corresponding change to increases to pensions in payment, increases to pensions in deferment and increases in salary is recognised. 33. Sensitivities continued (b) Sensitivities on financial instruments We are further required to show additional sensitivity analysis under IFRS 7 and these are shown separately in the subsequent table due to the additional assumptions that are made in order to produce meaningful sensitivity disclosures. Our financial instruments are sensitive to changes in market variables, being UK and US interest rates, the UK RPI and the dollar to sterling exchange rate. The changes in market variables impact the valuation of our borrowings, deposits, derivative financial instruments and commodity contract derivatives. The analysis illustrates the sensitivity of our financial instruments to reasonably possible changes in market variables. The following main assumptions were made in calculating the sensitivity analysis for continuing operations: • the amount of net debt, the ratio of fixed to floating interest rates of the debt and derivatives portfolio, and the proportion of financial instruments in foreign currencies are all constant and on the basis of the hedge designations in place at 31 March 2018 and 2017 respectively; • the statement of financial position sensitivity to interest rates relates only to derivative financial instruments and available-for-sale investments, as debt and other deposits are carried at amortised cost and so their carrying value does not change as interest rates move; • the sensitivity of accrued interest to movements in interest rates is calculated on net floating rate exposures on debt, deposits and derivative instruments; • changes in the carrying value of derivatives from movements in interest rates of designated cash flow hedges are assumed to be recorded fully within equity; and • changes in the carrying value of derivative financial instruments designated as net investment hedges from movements in interest rates are recorded in the income statement as they are designated using the spot rather than the forward translation method. The impact of movements in the dollar to sterling exchange rate are recorded directly in equity. 2018 2017 Income statement £m Other equity reserves £m Income statement £m Other equity reserves £m Financial risk (post-tax) 1 : UK RPI change of 0.5% 2 30 — 28 — UK interest rates change of 0.5% 43 26 64 35 US interest rates change of 0.5% 39 17 61 22 US dollar exchange rate change of 10% 3 48 479 46 604 1. Excludes sensitivities to the Further Acquisition Agreement derivative. Further details on sensitivities are provided in note 30(f). 2. Excludes sensitivities to LPI curve. Further details on sensitivities are provided in note 30(f). 3. The other equity reserves impact does not reflect the exchange translation in our US subsidiaries’ net assets. It is estimated this would change by £1,040 million ( 2017 : £988 million ) in the opposite direction if the dollar exchange rate changed by 10% . Additional sensitivities in respect to commodity price risk and to our derivative fair values are as follows: 2018 2017 Income statement £m Net assets £m Income statement £m Net assets £m Commodity risk 1 (post-tax): 10% increase in commodity prices 23 23 28 28 10% decrease in commodity prices (23 ) (23 ) (29 ) (29 ) Assets and liabilities carried at fa

Borrowing facilities

Borrowing facilities	12 Months Ended
	Mar. 31, 2018
Disclosure of detailed information about borrowings [abstract]
Borrowing facilities	Borrowing facilities To support our long-term liquidity requirements and provide backup to commercial paper and other borrowings, we agree loan facilities with financial institutions over and above the value of borrowings that may be required. These facilities have never been drawn, and our undrawn amounts are listed below. At 31 March 2018 , we had bilateral committed credit facilities of £5,438 million ( 2017 : £4,938 million ). In addition, we had committed credit facilities from syndicates of banks of £245 million at 31 March 2018 ( 2017 : £338 million ). All committed credit facilities were undrawn in 2018 and 2017 . An analysis of the maturity of these undrawn committed facilities is shown below: 2018 2017 £m £m Undrawn committed borrowing facilities expiring: Less than 1 year — — In 1 to 2 years 3,910 1,115 In 2 to 3 years — 2,388 In 3 to 4 years — — In 4 to 5 years 1,773 1,773 More than 5 years — — 5,683 5,276 Of the unused facilities at 31 March 2018 , £5,438 million ( 2017 : £4,938 million ) is available for liquidity purposes, while £245 million ( 2017 : £338 million ) is available as backup to specific US borrowings. In addition to the above, the Group has Export Credit Agreements (ECAs) totalling £797 million ( 2017 : £600 million ), of which £704 million ( 2017 : £562 million ) is undrawn. During the year, two new ECAs totalling £261 million were made available, of which £239 million is undrawn. Of the £3,910 million of undrawn committed borrowing facilities due to expire within 1 to 2 years, £1,255 million was renegotiated before 31 March 2018, with effect from 1 June 2018. This amount has increased to £1,380 million with the expiry extended to more than 5 years. Further information on our bonds can be found on the debt investor section of our website. Unless included in these financial statements the information on our website is unaudited.

Subsidiary undertakings, joint

Subsidiary undertakings, joint ventures and associates	12 Months Ended
	Mar. 31, 2018
Interests In Other Entities [Abstract]
Subsidiary undertakings, joint ventures and associates	Investments in joint ventures and associates Investments in joint ventures and associates represent businesses we do not control, but instead exercise joint control or significant influence. A joint venture is an arrangement established to engage in economic activity, which the Group jointly controls with other parties and has rights to the net assets of the arrangement. An associate is an entity which is neither a subsidiary nor a joint venture, but over which the Group has significant influence. 2018 2017 Associates £m Joint ventures £m Total £m Associates £m Joint ventures £m Total £m Share of net assets at 1 April 1,776 307 2,083 84 313 397 Exchange adjustments (19 ) 7 (12 ) 16 19 35 Additions 65 64 129 74 63 137 Acquisition of stake in Quadgas HoldCo Limited — — — 1,611 — 1,611 Capitalisation of shareholder loan to Quadgas HoldCo Limited 69 — 69 — — — Impairment charge against investment in Quadgas HoldCo Limited (213 ) — (213 ) — — — Share of post-tax results for the year 147 26 173 15 48 63 Share of other comprehensive income of associates, net of tax 147 — 147 — — — Dividends received (170 ) (43 ) (213 ) (24 ) (75 ) (99 ) Other movements 5 — 5 — (61 ) (61 ) Share of net assets at 31 March 1,807 361 2,168 1,776 307 2,083 A list of joint ventures and associates including the name and proportion of ownership is provided in note 32. Further information on the Group’s acquisition of an associate interest in Quadgas HoldCo Limited is provided in note 9 and opposite. On 1 May 2018, the Group announced that it had entered into the Remaining Acquisition Agreement (RAA) with Quadgas Investments BidCo Limited (the Consortium) regarding the potential sale of its remaining 25% interest in Quadgas HoldCo Limited, the holding company for Cadent Gas Limited. The RAA contains both put and call options for both the Group and the Consortium that can be exercised in the period between 1 March 2019 and 31 October 2019 (subject to giving six months’ notice) for cash proceeds of approximately £1.2 billion . The nature of the contractual arrangements in the RAA are substantially the same as those within the Further Acquisition Agreement (FAA), which also contains put and call options that can be exercised by either party during the same time-frame. The carrying value of the Group’s equity interest in Quadgas HoldCo Limited at 31 March 2017 was £1.6 billion , reflecting our best estimate of the fair value of that stake, as it was treated as an asset acquired at fair value following the disposal of a controlling interest in the UK Gas Distribution business. As set out in note 16, at 31 March 2018 we recognised a derivative financial asset of £110 million relating to the FAA agreement. In addition, an impairment review of the Group’s interests in Quadgas HoldCo Limited, (comprising the FAA derivative, a 39% equity interest and £352 million in shareholder loans), was undertaken, comparing the aggregated carrying value of these interests against the future dividend income and proceeds we would expect to receive under the FAA and RAA. This resulted in a charge of £213 million , recorded as impairment against the carrying value of the equity. The impairment largely offsets pension accounting gains, and the recognition of the FAA derivative asset. Following recognition of the FAA derivative asset and the impairment charge, the aggregate carrying value of Group’s interests associated with Quadgas HoldCo Limited is £2.1 billion , which is close to our original assessment of fair market value as determined at 31 March 2017. Of this, £1.6 billion relates to our equity interest. In 2017 the Group first entered into an arrangement with San Francisco-based Sunrun Neptune Investor 2016 LLC, a leading US provider of residential solar energy systems to provide investment capital. In the period to 31 March 2018 , the Group invested £38 million ( 2017 : £41 million ) alongside Sunrun into a newly incorporated partnership vehicle. The investment is classified as an associate as the Group has significant influence over the activities of the partnership vehicle. The fair value gain on this investment of £7 million has been reflected within the share of post-tax results for the year. The joint ventures and associates have no significant contingent liabilities to which the Group is exposed, and the Group has no significant contingent liabilities in relation to its interests in the joint ventures and associates. The Group has capital commitments of £120 million ( 2017 : £235 million ) in relation to joint ventures. Outstanding balances with joint ventures and associates are shown in note 29. At 31 March 2018 , the Group had one material joint venture, being its 50% equity stake in BritNed Development Limited, and one material associate being its 39% equity stake in Quadgas HoldCo Limited. 15. Investments in joint ventures and associates continued BritNed Development Limited (joint venture) BritNed Development Limited is a joint venture with transmission system operator TenneT and operates the subsea electricity link between Great Britain and the Netherlands, commissioned in 2011 . BritNed Development Limited has a reporting period end of 31 December with monthly management reporting information provided to National Grid. Summarised financial information of this joint venture, as at 31 March, together with the carrying amount of the investment in the consolidated financial statements is as follows: 2018 2017 £m £m Statement of financial position – BritNed Development Limited Non-current assets 390 392 Cash and cash equivalents 50 45 All other current assets 4 1 Non-current liabilities (10 ) (10 ) Current liabilities (28 ) (20 ) Equity 406 408 Carrying amount of the Group’s investment (National Grid ownership 50%) 203 204 2018 2017 £m £m Income statement – BritNed Development Limited Revenue 429 399 Depreciation and amortisation (13 ) (13 ) Other costs (324 ) (257 ) Operating profit 92 129 Income tax expense (20 ) (23 ) Profit for the year 72 106 Group’s share of profit (National Grid ownership 50%) 36 53 Quadgas HoldCo Limited (associate) As set out in note 9, on disposal of the Group’s interest in the UK Gas Distribution business, the Group retained an equity interest in UK Gas Distribution through its parent, Quadgas HoldCo Limited, and a shareholder loan asset of £0.4 billion (see note 14). The Group has the power to appoint 4 of the 12 members of the board of Quadgas HoldCo Limited, which confers significant influence, but not joint control. In general, the key strategic, operational and financial decisions can be effected by a simple majority of votes. However, in limited circumstances, certain decisions require the consent of both parties. While these circumstances are not expected to occur regularly, given the rights conferred, and in view of the Group’s equity stake, the investment has been accounted for as an equity investment in an associate. The Group initially recognised its 39% interest in Quadgas HoldCo Limited at fair value, being the market price of the investment as at 31 March 2017 . The FAA was signed concerning a 14% interest in Quadgas HoldCo Limited, which contains put and call options for both the Group and the Consortium that can be exercised in the period between 1 March 2019 and 31 October 2019. The FAA is accounted for as a derivative financial instrument (see notes 16 and 30(f)). Quadgas HoldCo Limited is an unlisted entity, and so no quoted price exists. The fair value on initial recognition was determined with reference to the equity value of the business implicit in the sale transaction, adjusted to reflect a deduction for the estimated premium paid for control by the Consortium. In assigning value to the retained interests, the Group valued 14% of its 39% interest based on the price implied by the FAA. The deduction for control premium was applied to the residual 25% interest. 15. Investments in joint ventures and associates continued The Group has completed a purchase price allocation exercise for its interest in Quadgas HoldCo Limited. There were adjustments arising from the finalisation of the purchase price allocation. A £33 million loss was recognised relating to the completion accounts settlement, resulting in a £20 million payment being made to Quadgas BidCo Limited. The fair values of the assets and liabilities of Quadgas HoldCo Limited as at 31 March 2017 , as previously reported, are set out below, along with the reconciliation to the carrying value of the investment in the associate at that date: 2017 £m Statement of financial position – Quadgas HoldCo Limited (as previously reported) Non-current assets 15,559 Current assets 299 Non-current liabilities (10,408 ) Current liabilities (519 ) Equity 4,931 Proportion of the Group’s ownership interest in associate 1,923 Discount for non-controlling interest (312 ) Carrying amount of the Group’s interest in associate (National Grid ownership 39%) 1,611 Summarised financial information of this associate (incorporating the purchase price allocation completion adjustments), as at 31 March, together with the carrying amount of the investment in the consolidated financial statements is as follows: 2018 2017 £m £m Statement of financial position – Quadgas HoldCo Limited Non-current assets 16,735 16,047 Current assets 427 299 Non-current liabilities (11,195 ) (10,864 ) Current liabilities (534 ) (551 ) Equity 5,433 4,931 Proportion of the Group’s ownership interest in associate 2,119 1,923 Discount for non-controlling interest (arising on initial acquisition) (312 ) (312 ) Impairment charge against investment (213 ) — Carrying amount of the Group’s interest in associate (National Grid ownership 39%) 1,594 1,611 The sale of the previously owned subsidiary and subsequent acquisition of the 39% equity interest occurred (see note 9) on 31 March 2017 . All profit or loss impact for the year ended 31 March 2017 is disclosed as discontinued operations. The summarised income statement for the year ended 31 March 2018 is set out below: 2018 £m Income statement – Quadgas HoldCo Limited Revenue 1,468 Depreciation and amortisation (378 ) Other costs (423 ) Operating profit 667 Net interest payable (272 ) Income tax expense (76 ) Profit for the year 319 Group’s share of profit (National Grid ownership 39%) 124 Subsidiary undertakings, joint ventures and associates While we present consolidated results in these financial statements as if we were one company, our legal structure is such that there are a number of different operating and holding companies that contribute to the overall result. This structure has evolved through acquisitions as well as regulatory requirements to have certain activities within separate legal entities. Subsidiary undertakings A list of the Group’s subsidiaries as at 31 March 2018 is given below. The entire share capital of subsidiaries is held within the Group except where the Group’s ownership percentages are shown. These percentages give the Group’s ultimate interest and therefore allow for the situation where subsidiaries are owned by partly-owned intermediate subsidiaries. Where subsidiaries have different classes of shares, this is largely for historical reasons and the effective percentage holdings given represent both the Group’s voting rights and equity holding. Shares in National Grid (US) Holdings Limited, National Grid Holdings One plc and NGG Finance plc are held directly by National Grid plc. All other holdings in subsidiaries are owned by other subsidiaries within the Group. All subsidiaries are consolidated in the Group’s financial statements. Principal Group companies are identified in bold . These companies are incorporated and principally operate in the countries under which they are shown. Incorporated in England and Wales Registered office: 1–3 Strand, London WC2N 5EH, UK (unless stated otherwise in footnotes). Beegas Nominees Limited Birch Sites Limited Carbon Sentinel Limited Droylsden Metering Services Limited Gridcom Limited Icelink Interconnector Limited Landranch Limited Lattice Group Employee Benefit Trust Limited Lattice Group Limited Lattice Group Trustees Limited Natgrid Limited NatGrid One Limited NatgridTW1 Limited National Grid Belgium Limited National Grid Blue Power Limited National Grid Carbon Limited National Grid Commercial Holdings Limited National Grid Distributed Energy Limited National Grid Electricity Group Trustee Limited National Grid Electricity System Operator Limited National Grid Electricity Transmission plc National Grid Energy Metering Limited National Grid Four Limited National Grid Fourteen Limited National Grid Gas Holdings Limited National Grid Gas plc National Grid Grain LNG Limited National Grid Holdings Limited National Grid Holdings One plc National Grid IFA 2 Limited National Grid Interconnector Holdings Limited National Grid Interconnectors Limited National Grid International Limited National Grid Metering Limited National Grid North Sea Link Limited National Grid Offshore Limited (previously Cadent Services Limited)* National Grid Property Holdings Limited National Grid Seventeen Limited National Grid Smart Limited National Grid Ten National Grid Thirty Five Limited National Grid Thirty Four Limited (previously Cadent Gas Limited)* National Grid Thirty Six Limited (previously Cadent Finance Limited)* National Grid Twelve Limited National Grid Twenty Eight Limited National Grid Twenty-Five Limited National Grid Twenty Seven Limited National Grid Twenty Three Limited National Grid UK Limited National Grid UK Pension Services Limited National Grid (US) Holdings Limited National Grid (US) Investments 2 Limited National Grid (US) Investments 4 Limited National Grid (US) Partner 1 Limited National Grid Ventures Limited National Grid Viking Link Limited National Grid William Limited NG Nominees Limited NG Shetland Link Limited NGC Employee Shares Trustee Limited NGG Finance plc Ngrid Intellectual Property Limited NGT Telecom No. 1 Limited NGT Two Limited Port Greenwich Limited Stargas Nominees Limited Supergrid Electricity Limited Supergrid Energy Transmission Limited Supergrid Limited Thamesport Interchange Limited The National Grid Group Quest Trustee Company Limited The National Grid YouPlan Trustee Limited Transco Limited Warwick Technology Park Management Company (No 2) Limited ( 60.56% ) 1 1. Registered office: Shire Hall, PO Box 9, Warwick CV34 4RL, UK * Change of name effective from 2 May 2017 32. Subsidiary undertakings, joint ventures and associates continued Incorporated in the US Registered office: Corporation Service Company, 251 Little Falls Drive, Wilmington, DE 19808, New Castle County, USA (unless stated otherwise in footnotes). Boston Gas Company 1 British Transco Capital Inc. British Transco Finance, Inc. Broken Bridge Corp. 2 Colonial Gas Company 1 EUA Energy Investment Corporation 1 GridAmerica Holdings Inc. Grid NY LLC 3 KeySpan CI Midstream Limited KeySpan Corporation 3 KeySpan Energy Corporation 3 KeySpan Energy Services Inc. KeySpan Gas East Corporation 3 KeySpan International Corporation KeySpan MHK, Inc. KeySpan Midstream Inc. KeySpan Plumbing Solutions, Inc. 3 KSI Contracting, LLC KSI Electrical, LLC KSI Mechanical, LLC Land Management & Development, Inc. 3 Landwest, Inc. 3 Massachusetts Electric Company 1 Metro Energy, LLC 3 Metrowest Realty LLC Mystic Steamship Corporation Nantucket Electric Company 1 National Grid Algonquin LLC National Grid Connect Inc. National Grid Development Holdings Corp. National Grid Electric Services, LLC 3 National Grid Energy Management, LLC National Grid Energy Services LLC National Grid Energy Trading Services LLC 3 National Grid Engineering & Survey Inc. 3 National Grid Generation LLC 3 National Grid Generation Ventures LLC 3 National Grid Glenwood Energy Center, LLC National Grid Green Homes Inc. National Grid IGTS Corp. 3 National Grid Insurance USA Ltd 3 National Grid Islander East Pipeline LLC National Grid LNG GP LLC National Grid LNG LLC National Grid LNG LP LLC National Grid Millennium LLC National Grid NE Holdings 2 LLC 1 National Grid North America Inc. National Grid North East Ventures Inc. National Grid Port Jefferson Energy Center LLC National Grid Services Inc. National Grid Technologies Inc. 3 National Grid Transmission Services Corporation 1 National Grid US 6 LLC National Grid US LLC National Grid USA National Grid USA Service Company, Inc. 3 Nees Energy, Inc. 1 New England Electric Transmission Corporation 2 New England Energy Incorporated 1 New England Hydro Finance Company, Inc. ( 53.704% ) 1 New England Hydro-Transmission Corporation ( 53.704% ) 2 New England Hydro-Transmission Electric Company Inc. ( 53.704% ) 1 New England Power Company 1 Newport America Corporation 4 NGNE LLC Niagara Mohawk Energy, Inc. Niagara Mohawk Holdings, Inc. 3 Niagara Mohawk Power Corporation 3 NM Properties, Inc. 3 North East Transmission Co., Inc. Opinac North America, Inc. Philadelphia Coke Co., Inc. Port of the Islands North, LLC 3 The Brooklyn Union Gas Company 3 The Narragansett Electric Company 4 Transgas, Inc. 1 Upper Hudson Development Inc. 1 Valley Appliance and Merchandising Company 4 Vermont Green Line Devco, LLC ( 90% ) Wayfinder Group, Inc. 1 Incorporated in Australia Registered office: Level 7, 330 Collins Street, Melbourne, VIC 3000, Australia National Grid Australia Pty Limited Incorporated in Canada Registered office: 1959 Upper Water Street, Suite 800, Halifax NS, B3J 2X2, Canada KeySpan Energy Development Co. Incorporated in the Cayman Islands Registered office: c/o KPMG, PO Box 493, 2nd Floor, Century Yard, Cricket Square, Grand Cayman KY1-1106, Cayman Islands British Transco Finance (No 1) Limited* British Transco Finance (No 2) Limited* Incorporated in the Isle of Man Registered office: Third Floor, St George’s Court, Upper Church Street, Douglas, IM1 1EE, Isle of Man, UK (unless stated otherwise in footnotes) Lattice Telecom Finance (No 1) Limited 5 * National Grid Insurance Company (Isle of Man) Limited NGT Holding Company (Isle of Man) Limited Incorporated in Jersey Registered office: 44 Esplanade, St Helier, Jersey JE4 9WG, UK National Grid Jersey Investments Limited NG Jersey Limited Incorporated in the Netherlands Registered office: Westblaak 89, 3012 KG Rotterdam, PO Box 21153, 3001 AD, Rotterdam, Netherlands British Transco International Finance B.V. Registered office: Prins Bernhardplein 200, 1097 JB, Amsterdam, Netherlands National Grid Holdings B.V. Incorporated in the Republic of Ireland Registered office: Third Floor, The Metropolitan Building, James Joyce Street, Dublin 1, Ireland National Grid Insurance Company (Ireland) Designated Activity Company 1. Registered office: Corporation Service Company, 84 State Street, Boston MA 02109, Suffolk County, USA. 2. Registered office: Corporation Service Company, 10 Ferry Street, Suite 313, Concord NH 03301, Merrimack County, USA. 3. Registered office: Corporation Service Company, 80 State Street, Albany NY 12207-2543, Albany County, USA. 4. Registered office: Corporation Service Company, 222 Jefferson Boulevard, Suite 200, Warwick RI 02888, Kent County, USA. 5. Registered office: Heritage Court, 41 Athol Street, Douglas, IM99 1HN, Isle of Man, UK. * In liquidation. 32. Subsidiary undertakings, joint ventures and associates continued Joint ventures A list of the Group’s joint ventures as at 31 March 2018 is given below. All joint ventures are included in the Group’s financial statements using the equity method of accounting. Principal joint ventures are identified in bold . Incorporated in England and Wales Registered office: 1–3 Strand, London WC2N 5EH, UK (unless stated otherwise in footnotes). BritNed Development Limited ( 50% )* Joint Radio Company Limited ( 50% ) 1 ** Nemo Link Limited ( 50% ) NGET/SPT Upgrades Limited ( 50% ) † St William Homes LLP ( 50% ) 2 Incorporated in the US Registered office: Corporation Service Company, 251 Little Falls Drive, Wilmington, DE 19808, New Castle County, USA (unless stated otherwise in footnotes). Clean Energy Generation, LLC ( 50% ) Island Park Energy Center, LLC ( 50% ) Islander East Pipeline Company, LLC ( 50% ) 3 LI Energy Storage System, LLC ( 50% ) LI Solar Generation, LLC ( 50% ) Swan Lake North Holdings LLC ( 50% ) Incorporated in France Registered office: 1 Terrasse Bellini, Tour Initiale, TSA 41000 – 9291, Paris La Defense, CEDEX, France IFA2 SAS ( 50% ) Associates A list of the Group’s associates as at 31 March 2018 is given below. All associates are included in the Group’s financial statements using the equity method of accounting. Principal associates are identified in bold . Incorporated in England and Wales Registered office: Ashbrook Court, Prologis Park, Central Boulevard, Coventry CV7 8PE, UK Quadgas HoldCo Limited ( 39% ) Incorporated in the US Registered office: Corporation Service Company, 251 Little Falls Drive, Wilmington, DE 19808, New Castle County, USA (unless stated otherwise in footnotes). Algonquin Gas Transmission, LLC ( 20% ) 3 Clean Line Energy Partners LLC ( 32% ) Connecticut Yankee Atomic Power Company ( 19.5% ) 4 Direct Global Power, Inc. ( 26% ) 5 Energy Impact Fund LP ( 9.7% ) 6 KHB Venture LLC ( 33% ) 7 Maine Yankee Atomic Power Company ( 24% ) 8 Millennium Pipeline Company, LLC ( 26.25% ) 3 New York Transco LLC ( 28.3% ) 9 Nysearch RMLD, LLC ( 22.63% ) Sunrun Neptune Investor 2016 LLC 3 *** Yankee Atomic Electric Company ( 34.5% ) 10 Incorporated in Belgium Registered office: Avenue de Cortenbergh 71, 1000 Brussels, Belgium Coreso SA ( 15.84% ) Other investments A list of the Group’s other investments as at 31 March 2018 is given below. Incorporated in England and Wales Registered office: 1 More London Place, London SE1 2AF, UK Energis plc ( 33.06% ) ‡ 1. Registered office: Friars House, Manor House Drive, Coventry CV1 2TE, UK. 2. Registered office: Berkeley House, 19 Portsmouth Road, Cobham, Surrey KT11 1JG, UK. 3. Registered office: Corporation Trust Company, 1209 Orange, Wilmington DE 19808, New Castle County, USA. 4. Registered office: Carla Pizzella, 362 Injun Hollow Road, East Hampton CT 06424, USA. 5. Registered office: 507 Plum Street, PO Box 5001, Syracuse NY 13250, USA. 6. Registered office: Harvard Business Services, Inc., 16192 Coastal Highway, Lewes DE 19958, Sussex County, USA. 7. Registered office: De Maximus Inc., 135 Beaver Street, 4th Floor, Waltham MA 02452, USA. 8. Registered office: Joseph D Fay, 321 Old Ferry Road, Wiscasset ME 04578, USA. 9. Registered office: Corporation Service Company, 80 State Street, Albany NY 12207, USA. 10. Registered office: Brian Smith, 49 Yankee Road, Rowe MA 01367, USA. * National Grid Interconnector Holdings Limited owns 284,500,000 €0.20 C Ordinary shares and one £1.00 Ordinary A share. ** National Grid Gas plc owns all £1.00 A Ordinary shares. *** National Grid Green Homes Inc owns 1,000 Class A Membership Interests. † National Grid Electricity Transmission plc owns 50 £1.00 A Ordinary shares. ‡ In administration. Our interests and activities are held or operated through the subsidiaries, joint arrangements or associates as disclosed above. These interests and activities (and their branches) are established in – and subject to the laws and regulations of – these jurisdictions.

Sensitivites

Sensitivites	12 Months Ended
	Mar. 31, 2018
Disclosure of sensitivity analysis for actuarial assumptions [abstract]
Sensitivities	Financial risk management Our activities expose us to a variety of financial risks including currency risk, interest rate risk, commodity price risk, credit risk, capital risk and liquidity risk. Our risk management programme focuses on the unpredictability of financial markets and seeks to minimise potential volatility of financial performance from these risks. We use financial instruments, including derivative financial instruments, to manage risks of this type. This note describes our approach to managing risk, including an analysis of assets and liabilities by currency type and an analysis of interest rate category for our net debt. We are required by accounting standards to also include a number of specific disclosures (such as a maturity analysis of contractual undiscounted cash flows) and have included these requirements below. Risk management related to financing activities is carried out by a central treasury department under policies approved by the Finance Committee of the Board. The objective of the treasury department is to manage funding and liquidity requirements, including managing associated financial risks, to within acceptable boundaries. The Finance Committee provides written principles for overall risk management, as well as written policies covering specific areas such as foreign exchange risk, interest rate risk, credit risk, liquidity risk, use of derivative financial instruments and non-derivative financial instruments, and investment of excess liquidity. We have exposure to the following risks, which are described in more detail below: • credit risk; • liquidity risk; • interest rate risk; • currency risk; and • capital risk. 30. Financial risk management continued (a) Credit risk We are exposed to the risk of loss resulting from counterparties’ default on their commitments including failure to pay or make a delivery on a contract. This risk is inherent in our commercial business activities. We are exposed to credit risk on our cash and cash equivalents, derivative financial instruments, deposits with banks and financial institutions, as well as credit exposures to wholesale and retail customers, including outstanding receivables and committed transactions. Treasury credit risk Counterparty risk arises from the investment of surplus funds and from the use of derivative financial instruments. As at 31 March 2018, the following limits were in place for investments held with banks and financial institutions: Maximum limit Long-term limit Triple ‘A’ G7 sovereign entities (AAA) 1,853 927 Triple ‘A’ vehicles (AAA) 500 — Triple ‘A’ range institutions and non G7 sovereign entities (AAA) 1,011 506 Double ‘A+’ G7 sovereign entities (AA+) 1,685 843 Double ‘A’ range institutions (AA) 674 to 843 337 to 421 Single ‘A’ range institutions (A) 236 to 337 118 to 169 The maximum limit applies to all transactions, including long-term transactions. The long-term limit applies to transactions which mature in more than 12 months ’ time. As at 31 March 2017 and 2018, we had a number of exposures to individual counterparties. In accordance with our treasury policies, counterparty credit exposure utilisations are monitored daily against the counterparty credit limits. Counterparty credit ratings and market conditions are reviewed continually with limits being revised and utilisation adjusted, if appropriate. Management does not expect any significant losses from non performance by these counterparties. Commodity credit risk The credit policy for US-based commodity transactions is owned by the Finance Committee to the Board, which establishes controls and procedures to determine, monitor and minimise the credit exposure to counterparties. Authority to administer the policy has been delegated to the Energy Procurement Risk Management Committee by the Board and the Executive Committee. Wholesale and retail credit risk Our principal commercial exposure in the UK is governed by the credit rules within the regulated codes: Uniform Network Code and Connection and Use of System Code. These set out the level of credit relative to the RAV for each credit rating. In the US, we are required to supply electricity and gas under state regulations. Our credit policies and practices are designed to limit credit exposure by collecting security deposits prior to providing utility services, or after utility service has commenced if certain applicable regulatory requirements are met. Collection activities are managed on a daily basis. Sales to retail customers are usually settled in cash, cheques, electronic bank payments or by using major credit cards. We are committed to measuring, monitoring, minimising and recording counterparty credit risk in our wholesale business. The utilisation of credit limits is regularly monitored and collateral is collected against these accounts when necessary. Management does not expect any significant losses of receivables that have not been provided for as shown in note 18. 30. Financial risk management continued (a) Credit risk continued Offsetting financial assets and liabilities The following tables set out our financial assets and liabilities which are subject to offset and to enforceable master netting arrangements or similar agreements. The tables show the amounts which are offset and reported net in the statement of financial position. Amounts which cannot be offset under IFRS, but which could be settled net under terms of master netting agreements if certain conditions arise, and with collateral received or pledged, are shown to present National Grid’s net exposure. Financial assets and liabilities on different transactions are only reported net if the transactions are with the same counterparty, a currently enforceable legal right of offset exists and the cash flows are intended to be settled on a net basis. Amounts which do not meet the criteria for offsetting on the statement of financial position, but could be settled net in certain circumstances, principally relate to derivative transactions under ISDA agreements where each party has the option to settle amounts on a net basis in the event of default of the other party. Commodity contract derivatives that have not been offset on the balance sheet may be settled net in certain circumstances under ISDA or NAESB (North American Energy Standards Board) agreements. National Grid has similar arrangements in relation to bank account balances and bank overdrafts, and trade payables and trade receivables which are subject to general terms and conditions. However, these balances are immaterial. Related amounts At 31 March 2018 Gross £m Gross amounts offset £m Net amount £m Financial instruments £m Cash £m Net amount £m Assets Financing derivatives 1,545 — 1,545 (523 ) (772 ) 250 Commodity contract derivatives 69 — 69 (16 ) — 53 Further Acquisition Agreement derivative 1 110 — 110 — — 110 1,724 — 1,724 (539 ) (772 ) 413 Liabilities Financing derivatives (945 ) — (945 ) 523 326 (96 ) Commodity contract derivatives (116 ) — (116 ) 16 7 (93 ) (1,061 ) — (1,061 ) 539 333 (189 ) 663 — 663 — (439 ) 224 1. The Group held a put/call option as at 31 March 2017. The fair value was £nil. Related amounts At 31 March 2017 Gross carrying amounts £m Gross amounts offset £m Net amount presented in statement of financial position £m Financial instruments £m Cash collateral received/ pledged £m Net amount £m Assets Financing derivatives 1,707 — 1,707 (718 ) (692 ) 297 Commodity contract derivatives 106 — 106 (25 ) — 81 1,813 — 1,813 (743 ) (692 ) 378 Liabilities Financing derivatives (2,223 ) — (2,223 ) 718 1,230 (275 ) Commodity contract derivatives (170 ) — (170 ) 25 18 (127 ) (2,393 ) — (2,393 ) 743 1,248 (402 ) (580 ) — (580 ) — 556 (24 ) 30. Financial risk management continued (b) Liquidity risk Our policy is to determine our liquidity requirements by the use of both short-term and long-term cash flow forecasts. These forecasts are supplemented by a financial headroom analysis which is used to assess funding requirements for at least a 24-month period and maintain adequate liquidity for a continuous 12 -month period. We believe our contractual obligations, including those shown in commitments and contingencies in note 28 can be met from existing cash and investments, operating cash flows and other financings that we reasonably expect to be able to secure in the future, together with the use of committed facilities if required. Our debt agreements and banking facilities contain covenants, including those relating to the periodic and timely provision of financial information by the issuing entity and financial covenants such as restrictions on the level of subsidiary indebtedness. Failure to comply with these covenants, or to obtain waivers of those requirements, could in some cases trigger a right, at the lender’s discretion, to require repayment of some of our debt and may restrict our ability to draw upon our facilities or access the capital markets. The following is an analysis of the contractual undiscounted cash flows payable under financial liabilities and derivative assets and liabilities as at the reporting date: At 31 March 2018 Less than £m 1 to 2 £m 2 to 3 £m More than £m Total £m Non-derivative financial liabilities Borrowings, excluding finance lease liabilities (4,099 ) (1,642 ) (2,325 ) (18,023 ) (26,089 ) Interest payments on borrowings 1 (730 ) (692 ) (629 ) (12,897 ) (14,948 ) Finance lease liabilities (60 ) (60 ) (45 ) (148 ) (313 ) Other non interest-bearing liabilities (2,840 ) (359 ) — — (3,199 ) Derivative financial liabilities Derivative contracts – receipts 1,069 601 130 250 2,050 Derivative contracts – payments (890 ) (263 ) (188 ) (529 ) (1,870 ) Commodity contract derivatives (80 ) (33 ) (26 ) 1 (138 ) (7,630 ) (2,448 ) (3,083 ) (31,346 ) (44,507 ) At 31 March 2017 Less than 1 year £m 1 to 2 years £m 2 to 3 years £m More than 3 years £m Total £m Non-derivative financial liabilities Borrowings, excluding finance lease liabilities (5,142 ) (1,864 ) (1,750 ) (19,245 ) (28,001 ) Interest payments on borrowings 1 (767 ) (707 ) (670 ) (12,975 ) (15,119 ) Finance lease liabilities (66 ) (58 ) (61 ) (183 ) (368 ) Other non interest-bearing liabilities (2,989 ) (260 ) — — (3,249 ) Derivative financial liabilities Derivative contracts – receipts 571 961 572 234 2,338 Derivative contracts – payments (1,551 ) (959 ) (304 ) (610 ) (3,424 ) Commodity contract derivatives (15 ) (18 ) (8 ) — (41 ) (9,959 ) (2,905 ) (2,221 ) (32,779 ) (47,864 ) 1. The interest on borrowings is calculated based on borrowings held at 31 March without taking account of future issues. Floating rate interest is estimated using a forward interest rate curve as at 31 March. Payments are included on the basis of the earliest date on which the Company can be required to settle. (c) Interest rate risk National Grid’s interest rate risk arises from our long-term borrowings. Borrowings issued at variable rates expose National Grid to cash flow interest rate risk, partially offset by cash held at variable rates. Borrowings issued at fixed rates expose National Grid to fair value interest rate risk. Our interest rate risk management policy is to seek to minimise total financing costs (being interest costs and changes in the market value of debt) subject to constraints. We do this by using fixed and floating rate debt and derivative financial instruments including interest rate swaps, swaptions and forward rate agreements. We hold some borrowings on issue that are inflation linked. We believe that these provide a partial economic offset to the inflation risk associated with our UK inflation linked revenues. The table in note 20 sets out the carrying amount, by contractual maturity, of borrowings that are exposed to interest rate risk before taking into account interest rate swaps. 30. Financial risk management continued (c) Interest rate risk continued During 2018 and 2017, net debt was managed using derivative financial instruments to hedge interest rate risk as follows: 2018 2017 Fixed rate £m Floating rate £m Inflation linked £m Other 1 £m Total £m Fixed rate £m Floating rate £m Inflation linked £m Other 1 £m Total £m Cash and cash equivalents — 302 — 27 329 940 199 — — 1,139 Financial investments 31 2,625 — 38 2,694 44 8,691 — 6 8,741 Borrowings (16,144 ) (3,191 ) (7,290 ) — (26,625 ) (17,681 ) (3,917 ) (7,040 ) — (28,638 ) Pre-derivative position (16,113 ) (264 ) (7,290 ) 65 (23,602 ) (16,697 ) 4,973 (7,040 ) 6 (18,758 ) Derivative effect 1,735 (1,237 ) 102 — 600 1,424 (1,785 ) (155 ) — (516 ) Net debt position (14,378 ) (1,501 ) (7,188 ) 65 (23,002 ) (15,273 ) 3,188 (7,195 ) 6 (19,274 ) 1. Represents financial instruments which are not directly affected by interest rate risk, such as investments in equity or other similar financial instruments. (d) Currency risk National Grid operates internationally and is exposed to foreign exchange risk arising from various currency exposures, primarily with respect to the US dollar. Foreign exchange risk arises from future commercial transactions, recognised assets and liabilities, and investments in foreign operations. Our policy for managing foreign exchange transaction risk is to hedge contractually committed foreign currency cash flows over a prescribed minimum size. Where foreign currency cash flow forecasts are less certain, our policy is to hedge a proportion of such cash flows. Instruments used to manage foreign exchange transaction risk include foreign exchange forward contracts and foreign exchange swaps. Our policy for managing foreign exchange translation risk relating to our net investment in foreign operations is to maintain cross-currency swaps and foreign exchange forwards so as to provide an economic offset. The primary managed foreign exchange exposure arises from the dollar denominated assets and liabilities held by our US operations, with a further small euro exposure in respect of joint venture investments. During 2018 and 2017, derivative financial instruments were used to manage foreign currency risk as follows: 2018 2017 Sterling £m Euro £m Dollar £m Other £m Total £m Sterling £m Euro £m Dollar £m Other £m Total £m Cash and cash equivalents 294 2 33 — 329 1,110 — 29 — 1,139 Financial investments 1,471 69 1,125 29 2,694 6,824 98 1,753 66 8,741 Borrowings (10,912 ) (3,794 ) (11,068 ) (851 ) (26,625 ) (11,099 ) (5,373 ) (10,729 ) (1,437 ) (28,638 ) Pre-derivative position (9,147 ) (3,723 ) (9,910 ) (822 ) (23,602 ) (3,165 ) (5,275 ) (8,947 ) (1,371 ) (18,758 ) Derivative effect 3,748 3,793 (7,992 ) 1,051 600 2,310 6,241 (10,708 ) 1,641 (516 ) Net debt position (5,399 ) 70 (17,902 ) 229 (23,002 ) (855 ) 966 (19,655 ) 270 (19,274 ) The exposure to dollars largely relates to our net investment hedge activities; exposure to euros largely relates to hedges for our future non-sterling capital expenditure as described in note 16. The currency exposure on other financial instruments is as follows: 2018 2017 Sterling £m Euro £m Dollar £m Other £m Total £m Sterling £m Euro £m Dollar £m Other £m Total £m Trade and other receivables 253 — 1,528 — 1,781 83 — 1,660 — 1,743 Trade and other payables (1,124 ) — (1,793 ) — (2,917 ) (1,209 ) — (1,795 ) — (3,004 ) Other non-current liabilities (144 ) — (254 ) — (398 ) (100 ) — (315 ) — (415 ) The carrying amounts of other financial instruments are denominated in the above currencies, which in most instances are the functional currency of the respective subsidiaries. Our exposure to dollars is due to activities in our US subsidiaries. We do not have any other significant exposure to currency risk on these balances. 30. Financial risk management continued (e) Capital risk management The capital structure of the Group consists of shareholders’ equity, as disclosed in the consolidated statement of changes in equity, and net debt (note 27). National Grid’s objectives when managing capital are: to safeguard our ability to continue as a going concern; to remain within regulatory constraints of our regulated operating companies; and to maintain an efficient mix of debt and equity funding thus achieving an optimal capital structure and cost of capital. We regularly review and manage the capital structure as appropriate in order to achieve these objectives. Maintaining appropriate credit ratings for our operating and holding companies is an important aspect of our capital risk management strategy and balance sheet efficiency. As noted on page 24, we monitor our balance sheet efficiency using several metrics including retained cash flow/net debt (RCF), interest cover and gearing. Retained cash flow/net debt and interest cover for the Group for the year ended 31 March 2018 were 9.7% ( 2017 : 14.9% ) and 4.4 ( 2017 : 5.0 ) respectively. We believe these are consistent with the current credit ratings for National Grid plc and the main operating companies within the Group, based on guidance from the rating agencies. We monitor the RAV gearing within each of NGET and the regulated transmission businesses within NGG. This is calculated as net debt expressed as a percentage of RAV, and indicates the level of debt employed to fund our UK regulated businesses. It is compared with the level of RAV gearing indicated by Ofgem as being appropriate for these businesses, at around 60 % to 62.5% . We also monitor net debt as a percentage of rate base for our US operating companies, comparing this with the allowed rate base gearing inherent within each of our agreed rate plans. The majority of our regulated operating companies in the US and the UK are subject to certain restrictions on the payment of dividends by administrative order, contract and/or licence. The types of restrictions that a company may have that would prevent a dividend being declared or paid unless they are met include: • dividends must be approved in advance by the relevant US state regulatory commission; • the subsidiary must have at least two recognised rating agency credit ratings of at least investment grade; • dividends must be limited to cumulative retained earnings, including pre-acquisition retained earnings; • National Grid plc must maintain an investment grade credit rating and if that rating is the lowest investment grade bond rating it cannot have a negative watch/review for downgrade notice by a credit rating agency; • the subsidiary must not carry on any activities other than those permitted by the licences; • the subsidiary must not create any cross-default obligations or give or receive any intra-group cross-subsidies; and • the percentage of equity compared with total capital of the subsidiary must remain above certain levels. There is a further restriction relating only to the Narragansett Electric Company, which is required to maintain its consolidated net worth above certain levels. These restrictions are subject to alteration in the US as and when a new rate case or rate plan is agreed with the relevant regulatory bodies for each operating company and in the UK through the normal licence review process. As most of our business is regulated, at 31 March 2018 the majority of our net assets are subject to some of the restrictions noted above. These restrictions are not considered to be significantly onerous, nor do we currently expect they will prevent the planned payment of dividends in future in line with our dividend policy. Some of our regulatory and bank loan agreements additionally impose lower limits for the long-term credit ratings that certain companies within the Group must hold. All the above requirements are monitored on a regular basis in order to ensure compliance. The Group has complied with all externally imposed capital requirements to which it is subject. (f) Fair value analysis Included in the statement of financial position are financial instruments which have been measured at fair value. These fair values can be categorised into hierarchy levels that are representative of the inputs used in measuring the fair value. The best evidence of fair value is a quoted price in an actively traded market. In the event that the market for a financial instrument is not active, a valuation technique is used. 2018 2017 Level 1 £m Level 2 £m Level 3 £m Total £m Level 1 £m Level 2 £m Level 3 £m Total £m Assets Investments – available-for-sale 2,406 310 5 2,721 7,717 315 5 8,037 Investments – joint ventures and associates — — 79 79 — — 41 41 Financing derivatives — 1,544 1 1,545 — 1,692 15 1,707 Commodity contract derivatives — 8 61 69 — 22 84 106 Further Acquisition Agreement derivative — — 110 110 — — — — 2,406 1,862 256 4,524 7,717 2,029 145 9,891 Liabilities Financing derivatives — (725 ) (220 ) (945 ) — (1,743 ) (480 ) (2,223 ) Commodity contract derivatives — (54 ) (62 ) (116 ) — (70 ) (100 ) (170 ) — (779 ) (282 ) (1,061 ) — (1,813 ) (580 ) (2,393 ) 2,406 1,083 (26 ) 3,463 7,717 216 (435 ) 7,498 Level 1: Financial instruments with quoted prices for identical instruments in active markets. Level 2: Financial instruments with quoted prices for similar instruments in active markets or quoted prices for identical or similar instruments in inactive markets, and financial instruments valued using models where all significant inputs are based directly or indirectly on observable market data. Level 3: Financial instruments valued using valuation techniques where one or more significant inputs are based on unobservable market data. 30. Financial risk management continued (f) Fair value analysis continued Our level 3 derivative financial instruments include cross-currency swaps, inflation linked swaps and equity options, all of which are traded on illiquid markets. In valuing these instruments we use in-house valuation models and obtain external valuations to support each reported fair value. Our level 3 commodity contract derivatives primarily consist of our forward purchases of electricity and gas where pricing inputs are unobservable, as well as other complex transactions. Complex transactions can introduce the need for internally developed models based on reasonable assumptions. Industry standard valuation techniques such as the Black-Scholes pricing model and Monte Carlo simulation are used for valuing such instruments. Level 3 is also applied in cases when optionality is present or where an extrapolated forward curve is considered unobservable. We consider forward curves to be unobservable if observed market transactions differ from the curve by more than 5% . Our level 3 investments include investments in associates relating to Sunrun Neptune Investor 2016 LLC, accounted at fair value (see note 15), and £5 million Series B preferred stocks in Enbala Holdings, Inc., accounted for as an available-for-sale investment. The Group is also party to the Further Acquisition Agreement (FAA) which contains a put/call option over 14% of the loan and equity it holds in Cadent (through its investment in Quadgas HoldCo Limited). The FAA is a derivative, which is accounted for at fair value, and the assumptions which are used to determine fair value are specific to the contract and not readily observable in active markets. The fair value of the option is £110 million ( 2017 : £ nil ). The changes in value of our level 3 derivative financial instruments are as follows: Financing derivatives/ Further Acquisition Agreement derivative Commodity contract derivatives Investments in associates and available-for-sale investments Total 2018 2017 2018 2017 2018 2017 2018 2017 £m £m £m £m £m £m £m £m At 1 April (465 ) (196 ) (16 ) (27 ) 46 — (435 ) (223 ) Net gains/(losses) for the year 1,2 125 (35 ) 8 (2 ) — — 133 (37 ) Purchases — — 27 15 41 46 68 61 Settlements 231 — (20 ) (2 ) (3 ) — 208 (2 ) Reclassification into level 3 — (234 ) — — — — — (234 ) At 31 March (109 ) (465 ) (1 ) (16 ) 84 46 (26 ) (435 ) 1. Gain of £125 million ( 2017 : £35 million loss) is attributable to derivative financial instruments held at the end of the reporting period and has been recognised in finance costs in the income statement. 2. Gain of £35 million ( 2017 : £21 million loss) is attributable to commodity contract derivative financial instruments held at the end of the reporting period. The impacts on a post-tax basis of reasonably possible changes in significant level 3 assumptions are as follows: Financing derivatives Commodity contract derivatives 2018 2017 2018 2017 £m £m £m £m 10% increase in commodity prices 1 — — (1 ) 1 10% decrease in commodity prices 1 — — 3 — Volume forecast uplift 2 — — — (1 ) Volume forecast reduction 2 — — — 1 +10% market area price change — — (6 ) (13 ) -10% market area price change — — 5 9 +20 basis points change in Limited Price Inflation (LPI) market curve 3 (84 ) (93 ) — — -20 basis points change in LPI market curve 3 82 88 — — 1. Level 3 commodity price sensitivity is included within the sensitivity analysis disclosed in note 33. 2. Volumes were flexed using maximum and minimum historical averages, or by > 10% where historical averages were not available. 3. A reasonably possible change in assumption of other level 3 derivative financial instruments is unlikely to result in a material change in fair values. The impacts disclosed above were considered on a contract by contract basis with the most significant unobservable inputs identified. The level 3 investments were acquired in the period on market terms and sensitivity is considered insignificant at 31 March 2018 . A 50 basis point change in the discount rate is used to determine the sensitivity of the fair value of our investment in Sunrun Neptune 2016 LLC. A 50 basis point increase/(decrease) would (decrease)/increase the fair value by £(5) million / £6 million (2017: £(8) million / £8 million ) respectively. A 5 percentage point decrease in the offer price received from Quadgas Investments BidCo Limited on 30 April 2018 applied in valuing the FAA would increase the fair value by approximately £39 million . Sensitivities In order to give a clearer picture of the impact on our results or financial position of potential changes in significant estimates and assumptions, the following sensitivities are presented. These sensitivities are hypothetical, as they are based on assumptions and conditions prevailing at the year-end, and should be used with caution. The effects provided are not necessarily indicative of the actual effects that would be experienced because our actual exposures are constantly changing. The sensitivities in the tables below show the potential impact in the income statement (and consequential impact on net assets) for a reasonably possible range of different variables each of which have been considered in isolation (i.e. with all other variables remaining constant). There are a number of these sensitivities which are mutually exclusive and therefore if one were to happen, another would not, meaning a total showing how sensitive our results are to these external factors is not meaningful. The sensitivities included in the tables below broadly have an equal and opposite effect if the sensitivity increases or decreases by the same amount unless otherwise stated. (a) Sensitivities on areas of estimation uncertainty The table below sets out the sensitivity analysis for each of the areas of estimation uncertainty set out in note 1E. These estimates are those that have a significant risk of resulting in a material adjustment to the carrying values of assets and liabilities in the next year. 2018 2017 Income statement £m Net assets £m Income statement £m Net assets £m Pensions and other post-retirement benefits 1 (pre-tax): UK discount rate change of 0.5% 2 8 1,075 9 1,305 US discount rate change of 0.5% 2 15 623 17 669 UK RPI rate change of 0.5% 3 5 965 8 1,114 UK long-term rate of increase in salaries change of 0.5% 4 — 61 2 80 US long-term rate of increase in salaries change of 0.5% 3 44 3 51 UK change of one year to life expectancy at age 65 2 588 2 673 US change of one year to life expectancy at age 65 4 359 4 365 Assumed US healthcare cost trend rates change of 1% 31 448 37 510 Environmental provision: 10% change in estimated future cash flows 154 154 175 175 0.5% change in discount rate 56 56 67 67 1. The changes shown are a change in the annual pension or other post-retirement benefit service charge and change in the defined benefit obligations. 2. A change in the discount rate is likely to occur as a result of changes in bond yields and as such would be expected to be offset to a significant degree by a change in the value of the bond assets held by the plans. 3. The projected impact resulting from a change in RPI reflects the underlying effect on pensions in payment, pensions in deferment and resultant increases in salary assumptions. 4. This change has been applied to both the pre 1 April 2014 and post 1 April 2014 rate of increase in salary assumption. Pensions and other post-retirement benefits assumptions Sensitivities have been prepared to show how the defined benefit obligations and annual service costs could potentially be impacted by changes in the relevant actuarial assumption that were reasonably possible as at 31 March 2018. In preparing sensitivities the potential impact has been calculated by applying the change to each assumption in isolation and assuming all other assumptions remain unchanged. This is with the exception of RPI in the UK where the corresponding change to increases to pensions in payment, increases to pensions in deferment and increases in salary is recognised. 33. Sensitivities continued (b) Sensitivities on financial instruments We are further required to show additional sensitivity analysis under IFRS 7 and these are shown separately in the subsequent table due to the additional assumptions that are made in order to produce meaningful sensitivity disclosures. Our financial instruments are sensitive to changes in market variables, being UK and US interest rates, the UK RPI and the dollar to sterling exchange rate. The changes in market variables impact the valuation of our borrowings, deposits, derivative financial instruments and commodity contract derivatives. The analysis illustrates the sensitivity of our financial instruments to reasonably possible changes in market variables. The following main assumptions were made in calculating the sensitivity analysis for continuing operations: • the amount of net debt, the ratio of fixed to floating interest rates of the debt and derivatives portfolio, and the proportion of financial instruments in foreign currencies are all constant and on the basis of the hedge designations in place at 31 March 2018 and 2017 respectively; • the statement of financial position sensitivity to interest rates relates only to derivative financial instruments and available-for-sale investments, as debt and other deposits are carried at amortised cost and so their carrying value does not change as interest rates move; • the sensitivity of accrued interest to movements in interest rates is calculated on net floating rate exposures on debt, deposits and derivative instruments; • changes in the carrying value of derivatives from movements in interest rates of designated cash flow hedges are assumed to be recorded fully within equity; and • changes in the carrying value of derivative financial instruments designated as net investment hedges from movements in interest rates are recorded in the income statement as they are designated using the spot rather than the forward translation method. The impact of movements in the dollar to sterling exchange rate are recorded directly in equity. 2018 2017 Income statement £m Other equity reserves £m Income statement £m Other equity reserves £m Financial risk (post-tax) 1 : UK RPI change of 0.5% 2 30 — 28 — UK interest rates change of 0.5% 43 26 64 35 US interest rates change of 0.5% 39 17 61 22 US dollar exchange rate change of 10% 3 48 479 46 604 1. Excludes sensitivities to the Further Acquisition Agreement derivative. Further details on sensitivities are provided in note 30(f). 2. Excludes sensitivities to LPI curve. Further details on sensitivities are provided in note 30(f). 3. The other equity reserves impact does not reflect the exchange translation in our US subsidiaries’ net assets. It is estimated this would change by £1,040 million ( 2017 : £988 million ) in the opposite direction if the dollar exchange rate changed by 10% . Additional sensitivities in respect to commodity price risk and to our derivative fair values are as follows: 2018 2017 Income statement £m Net assets £m Income statement £m Net assets £m Commodity risk 1 (post-tax): 10% increase in commodity prices 23 23 28 28 10% decrease in commodity prices (23 ) (23 ) (29 ) (29 ) Assets and liabilities carried at fa

Additional disclosures in respe

Additional disclosures in respect of guaranteed securities	12 Months Ended
	Mar. 31, 2018
Separate Financial Statements [Abstract]
Additional disclosures in respect of guaranteed securities	Additional disclosures in respect of guaranteed securities We have three debt issuances (including preferred shares) that are listed on a US national securities exchange and are guaranteed by other companies in the Group. These guarantors commit to honour any liabilities should the company issuing the debt have any financial difficulties. In order to provide debt holders with information on the financial stability of the companies providing the guarantees, we are required to disclose individual financial information for these companies. We have chosen to include this information in the Group financial statements rather than submitting separate stand-alone financial statements. The following condensed consolidating financial information, comprising statements of comprehensive income, statements of financial position and cash flow statements, is given in respect of National Grid Gas plc (subsidiary guarantor), which became joint full and unconditional guarantor on 11 May 2004 with National Grid plc (parent guarantor) of the 6.625% Guaranteed Notes due 2018 issued in June 1998 by British Transco Finance Inc., then known as British Gas Finance Inc. (issuer of notes). These Guaranteed Notes will be repaid on 1 June 2018. Condensed consolidating financial information is also provided in respect of Niagara Mohawk Power Corporation as a result of National Grid plc’s guarantee, dated 29 October 2007, of Niagara Mohawk’s 3.6% and 3.9% issued preferred shares. National Grid Gas plc, British Transco Finance Inc., and Niagara Mohawk Power Corporation are 100% owned and National Grid plc’s guarantee of Niagara Mohawk Power Corporation’s preferred shares is full and unconditional pursuant to Rule 3-10(i)(8) (i) and (ii) of Regulation S-X. The guarantees of National Grid Gas plc and National Grid plc are joint and several. The following financial information for National Grid plc, National Grid Gas plc, British Transco Finance Inc., and Niagara Mohawk Power Corporation on a condensed consolidating basis is intended to provide investors with meaningful and comparable financial information, and is provided pursuant to various rules including Rule 3-10 of Regulation S-X in lieu of the separate financial statements of each subsidiary issuer of public debt securities. This financial information should be read in conjunction with the other disclosures in these financial statements. Summary statements of comprehensive income are presented, on a consolidated basis, for the three years ended 31 March 2018 . Summary statements of comprehensive income of National Grid plc and National Grid Gas plc are presented under IFRS measurement principles, as modified by the inclusion of the results of subsidiary undertakings on the basis of equity accounting principles. The summary statements of financial position of National Grid plc and National Grid Gas plc include the investments in subsidiaries recorded on the basis of equity accounting principles for the purposes of presenting condensed consolidating financial information under IFRS. The summary statements of financial position present these investments within non-current financial and other investments. The consolidation adjustments column includes the necessary amounts to eliminate the intercompany balances and transactions between National Grid plc, National Grid Gas plc, British Transco Finance Inc., Niagara Mohawk Power Corporation and other subsidiaries. 34. Additional disclosures in respect of guaranteed securities continued Summary statements of comprehensive income for the year ended 31 March 2018 – IFRS Parent guarantor Issuer of notes Subsidiary guarantor National Grid plc £m Niagara Mohawk Power Corporation £m British Transco Finance Inc. £m National Grid Gas plc £m Other subsidiaries £m Consolidation adjustments £m National Grid consolidated £m Continuing operations Revenue — 2,416 — 1,430 11,495 (91 ) 15,250 Operating costs: Depreciation and amortisation — (190 ) — (245 ) (1,095 ) — (1,530 ) Payroll costs — (318 ) — (122 ) (1,208 ) — (1,648 ) Purchases of electricity — (537 ) — — (748 ) — (1,285 ) Purchases of gas — (166 ) — — (1,377 ) — (1,543 ) Rates and property tax — (183 ) — (94 ) (780 ) — (1,057 ) Balancing Service Incentive Scheme — — — — (1,012 ) — (1,012 ) Payments to other UK network owners — — — — (1,043 ) — (1,043 ) Other operating costs — (397 ) — (331 ) (2,002 ) 91 (2,639 ) — (1,791 ) — (792 ) (9,265 ) 91 (11,757 ) Total operating profit — 625 — 638 2,230 — 3,493 Net finance income/(costs) 889 (100 ) — (174 ) (1,360 ) — (745 ) Dividends receivable 950 — — — — (950 ) — Interest in equity accounted affiliates 1,672 — — 8 (40 ) (1,680 ) (40 ) Profit before tax 3,511 525 — 472 830 (2,630 ) 2,708 Tax 40 321 — (103 ) 626 — 884 Profit after tax from discontinued operations — — — 17 (58 ) — (41 ) Profit for the year 3,551 846 — 386 1,398 (2,630 ) 3,551 Amounts recognised in other comprehensive income from continuing operations 2 371 1 — 272 604 (877 ) 371 Total comprehensive income for the year 3,922 847 — 658 2,002 (3,507 ) 3,922 Attributable to: Equity shareholders 3,922 847 — 658 2,002 (3,507 ) 3,922 Non-controlling interests — — — — — — — 3,922 847 — 658 2,002 (3,507 ) 3,922 1. Profit for the year for British Transco Finance Inc. is £ nil as interest payable to external bond holders is offset by interest receivable on loans to National Grid Gas plc. 2. Includes other comprehensive income relating to interest in equity accounted affiliates. 34. Additional disclosures in respect of guaranteed securities continued Summary statements of comprehensive income for the year ended 31 March 2017 – IFRS Parent guarantor Issuer of notes Subsidiary guarantor National Grid plc £m Niagara Mohawk Power Corporation £m British Transco Finance Inc. £m National Grid Gas plc £m Other subsidiaries £m Consolidation adjustments £m National Grid consolidated £m Continuing operations Revenue — 2,388 — 1,376 11,435 (164 ) 15,035 Operating costs: Depreciation and amortisation — (193 ) — (256 ) (1,032 ) — (1,481 ) Payroll costs — (326 ) — (114 ) (1,138 ) — (1,578 ) Purchases of electricity — (511 ) — — (586 ) — (1,097 ) Purchases of gas — (140 ) — (67 ) (1,012 ) — (1,219 ) Rates and property tax — (188 ) — (101 ) (753 ) — (1,042 ) Balancing Service Incentive Scheme — — — — (1,120 ) — (1,120 ) Payments to other UK network owners — — — — (1,008 ) — (1,008 ) Other operating costs — (435 ) — (394 ) (2,617 ) 164 (3,282 ) — (1,793 ) — (932 ) (9,266 ) 164 (11,827 ) Total operating profit — 595 — 444 2,169 — 3,208 Net finance income/(costs) 8,177 (101 ) — (253 ) (8,910 ) — (1,087 ) Dividends receivable — — — — 8,100 (8,100 ) — Interest in equity accounted affiliates (401 ) — — — 63 401 63 Profit before tax 7,776 494 — 191 1,422 (7,699 ) 2,184 Tax 19 (181 ) — 16 (228 ) — (374 ) Profit after tax from discontinued operations — — — 4,633 1,351 — 5,984 Profit for the year 7,795 313 — 4,840 2,545 (7,699 ) 7,794 Amounts recognised in other comprehensive income from continuing operations 2 578 — — 114 177 (291 ) 578 Amounts recognised in other comprehensive income from discontinued operations 2 42 — — 51 (62 ) 11 42 Total comprehensive income for the year 8,415 313 — 5,005 2,660 (7,979 ) 8,414 Attributable to: Equity shareholders 8,415 313 — 5,005 2,661 (7,979 ) 8,415 Non-controlling interests — — — — (1 ) — (1 ) 8,415 313 — 5,005 2,660 (7,979 ) 8,414 1. Profit for the year for British Transco Finance Inc. is £ nil as interest payable to external bond holders is offset by interest receivable on loans to National Grid Gas plc. 2. Includes other comprehensive income relating to interest in equity accounted affiliates. 34. Additional disclosures in respect of guaranteed securities continued Summary statements of comprehensive income for the year ended 31 March 2016 – IFRS Parent guarantor Issuer of notes Subsidiary guarantor National Grid plc £m Niagara Mohawk Power Corporation £m British Transco Finance Inc. £m National Grid Gas plc re-presented 1 £m Other subsidiaries re-presented 1 £m Consolidation adjustments 1 £m National Grid consolidated £m Continuing operations Revenue — 2,027 — 1,244 10,069 (128 ) 13,212 Operating costs: Depreciation and amortisation — (162 ) — (255 ) (894 ) — (1,311 ) Payroll costs — (260 ) — (115 ) (962 ) — (1,337 ) Purchases of electricity — (484 ) — — (828 ) — (1,312 ) Purchases of gas — (86 ) — (75 ) (806 ) — (967 ) Rates and property tax — (155 ) — (101 ) (643 ) — (899 ) Balancing Service Incentive Scheme — — — — (907 ) — (907 ) Payments to other UK network owners — — — — (971 ) — (971 ) Other operating costs — (433 ) — (173 ) (1,805 ) 128 (2,283 ) — (1,580 ) — (719 ) (7,816 ) 128 (9,987 ) Total operating profit — 447 — 525 2,253 — 3,225 Net finance income/(costs) 701 (87 ) — (132 ) (1,437 ) — (955 ) Dividends receivable — — — — 620 (620 ) — Interest in equity accounted affiliates 1,843 — — 33 59 (1,876 ) 59 Profit before tax 2,544 360 — 426 1,495 (2,496 ) 2,329 Tax 47 (141 ) — (56 ) (277 ) — (427 ) Profit after tax from discontinued operations — — — 735 (43 ) — 692 Profit for the year 2,591 219 — 1,105 1,175 (2,496 ) 2,594 Amounts recognised in other comprehensive income from continuing operations 3 502 (1 ) — 8 426 (433 ) 502 Amounts recognised in other comprehensive income from discontinued operations 3 71 — — (13 ) 153 (140 ) 71 Total comprehensive income for the year 3,164 218 — 1,100 1,754 (3,069 ) 3,167 Attributable to: Equity shareholders 3,164 218 — 1,100 1,751 (3,069 ) 3,164 Non-controlling interests — — — — 3 — 3 3,164 218 — 1,100 1,754 (3,069 ) 3,167 1. Amounts have been re-presented to reflect the classification of the UK Gas Distribution business as a discontinued operation. 2. Profit for the year for British Transco Finance Inc. is £ nil as interest payable to external bond holders is offset by interest receivable on loans to National Grid Gas plc. 3. Includes other comprehensive income relating to interest in equity accounted affiliates. 34. Additional disclosures in respect of guaranteed securities continued Statements of financial position as at 31 March 2018 – IFRS Parent Issuer of notes Subsidiary guarantor National Grid plc £m Niagara Mohawk Power Corporation £m British Transco Finance Inc. £m National Grid Gas plc £m Other subsidiaries £m Consolidation adjustments £m National Grid consolidated £m Non-current assets Goodwill — 691 — — 4,753 — 5,444 Other intangible assets — 3 — 107 789 — 899 Property, plant and equipment — 6,148 — 4,433 29,272 — 39,853 Other non-current assets — 3 — 94 18 — 115 Amounts owed by subsidiary undertakings 350 — — 3,426 2,593 (6,369 ) — Pension assets — 231 — 412 766 — 1,409 Financial and other investments 21,708 30 — 101 12,340 (31,112 ) 3,067 Derivative financial assets 18 2 — 591 708 — 1,319 Total non-current assets 22,076 7,108 — 9,164 51,239 (37,481 ) 52,106 Current assets Inventories and current intangible assets — 36 — 22 283 — 341 Trade and other receivables 1 515 — 276 2,006 — 2,798 Current tax assets — — — — 307 (193 ) 114 Amounts owed by subsidiary undertakings 11,254 130 220 708 11,253 (23,565 ) — Financial and other investments 938 15 — 863 878 — 2,694 Derivative financial assets 308 7 — 46 — 44 405 Cash and cash equivalents — 4 — 271 54 — 329 Total current assets 12,501 707 220 2,186 14,781 (23,714 ) 6,681 Total assets 34,577 7,815 220 11,350 66,020 (61,195 ) 58,787 Current liabilities Borrowings (781 ) (51 ) (218 ) (675 ) (2,722 ) — (4,447 ) Derivative financial liabilities (187 ) (36 ) — (68 ) (66 ) (44 ) (401 ) Trade and other payables (62 ) (318 ) — (347 ) (2,726 ) — (3,453 ) Amounts owed to subsidiary undertakings (11,809 ) — — (624 ) (11,132 ) 23,565 — Current tax liabilities — (202 ) — (26 ) (88 ) 193 (123 ) Provisions — (23 ) — (66 ) (184 ) — (273 ) Total current liabilities (12,839 ) (630 ) (218 ) (1,806 ) (16,918 ) 23,714 (8,697 ) Non-current liabilities Borrowings (773 ) (2,087 ) — (3,635 ) (15,683 ) — (22,178 ) Derivative financial liabilities (41 ) (18 ) — (157 ) (444 ) — (660 ) Other non-current liabilities — (281 ) — (181 ) (855 ) — (1,317 ) Amounts owed to subsidiary undertakings (2,091 ) — — (500 ) (3,778 ) 6,369 — Deferred tax liabilities (1 ) (626 ) — (441 ) (2,568 ) — (3,636 ) Pensions and other post-retirement benefit obligations — (765 ) — — (907 ) — (1,672 ) Provisions — (248 ) — (129 ) (1,402 ) — (1,779 ) Total non-current liabilities (2,906 ) (4,025 ) — (5,043 ) (25,637 ) 6,369 (31,242 ) Total liabilities (15,745 ) (4,655 ) (218 ) (6,849 ) (42,555 ) 30,083 (39,939 ) Net assets 18,832 3,160 2 4,501 23,465 (31,112 ) 18,848 Equity Share capital 452 133 — 45 182 (360 ) 452 Share premium account 1,321 2,194 — 204 9,032 (11,430 ) 1,321 Retained earnings 21,599 830 2 2,929 14,217 (17,978 ) 21,599 Other equity reserves (4,540 ) 3 — 1,323 18 (1,344 ) (4,540 ) Shareholders’ equity 18,832 3,160 2 4,501 23,449 (31,112 ) 18,832 Non-controlling interests — — — — 16 — 16 Total equity 18,832 3,160 2 4,501 23,465 (31,112 ) 18,848 34. Additional disclosures in respect of guaranteed securities continued Statements of financial position as at 31 March 2017 – IFRS Parent Issuer of notes Subsidiary guarantor National Grid plc £m Niagara Mohawk Power Corporation 1 £m British Transco Finance Inc. £m National Grid Gas plc £m Other subsidiaries 1 £m Consolidation adjustments £m National Grid consolidated 1 £m Non-current assets Goodwill — 763 — — 5,333 — 6,096 Other intangible assets — — — 125 798 — 923 Property, plant and equipment — 6,553 — 4,358 28,914 — 39,825 Other non-current assets — 5 — 9 55 — 69 Amounts owed by subsidiary undertakings 342 — 239 3,426 2,576 (6,583 ) — Pension assets — 223 — 45 335 — 603 Financial and other investments 17,689 31 — 95 12,429 (27,061 ) 3,183 Derivative financial assets 149 2 — 813 603 — 1,567 Total non-current assets 18,180 7,577 239 8,871 51,043 (33,644 ) 52,266 Current assets Inventories and current intangible assets — 38 — 20 345 — 403 Trade and other receivables — 494 — 360 1,874 — 2,728 Current tax assets — — — — 317 — 317 Amounts owed by subsidiary undertakings 12,734 505 6 1,965 12,083 (27,293 ) — Financial and other investments 5,471 29 — 1,835 1,406 — 8,741 Derivative financial assets 202 13 — 51 174 (194 ) 246 Cash and cash equivalents 1,090 4 — (9 ) 54 — 1,139 Total current assets 19,497 1,083 6 4,222 16,253 (27,487 ) 13,574 Total assets 37,677 8,660 245 13,093 67,296 (61,131 ) 65,840 Current liabilities Borrowings (1,120 ) (55 ) (5 ) (833 ) (3,483 ) — (5,496 ) Derivative financial liabilities (533 ) (42 ) — (185 ) (581 ) 194 (1,147 ) Trade and other payables (46 ) (311 ) — (342 ) (2,646 ) — (3,345 ) Amounts owed to subsidiary undertakings (12,012 ) — — (2,151 ) (13,130 ) 27,293 — Current tax liabilities (3 ) (156 ) — (9 ) 61 — (107 ) Provisions — — — (184 ) (232 ) — (416 ) Total current liabilities (13,714 ) (564 ) (5 ) (3,704 ) (20,011 ) 27,487 (10,511 ) Non-current liabilities Borrowings (1,262 ) (2,345 ) (239 ) (3,879 ) (15,417 ) — (23,142 ) Derivative financial liabilities (272 ) (26 ) — (234 ) (714 ) — (1,246 ) Other non-current liabilities — (324 ) — (204 ) (842 ) — (1,370 ) Amounts owed to subsidiary undertakings (2,058 ) — — (756 ) (3,769 ) 6,583 — Deferred tax liabilities (3 ) (1,178 ) — (369 ) (2,929 ) — (4,479 ) Pensions and other post-retirement benefit obligations — (889 ) — — (1,647 ) — (2,536 ) Provisions — (298 ) — (104 ) (1,770 ) — (2,172 ) Total non-current liabilities (3,595 ) (5,060 ) (239 ) (5,546 ) (27,088 ) 6,583 (34,945 ) Total liabilities (17,309 ) (5,624 ) (244 ) (9,250 ) (47,099 ) 34,070 (45,456 ) Net assets 20,368 3,036 1 3,843 20,197 (27,061 ) 20,384 Equity Share capital 449 149 — 45 182 (376 ) 449 Share premium account 1,324 2,431 — 204 8,033 (10,668 ) 1,324 Retained earnings 22,582 456 1 2,268 11,914 (14,639 ) 22,582 Other equity reserves (3,987 ) — — 1,326 52 (1,378 ) (3,987 ) Shareholders’ equity 20,368 3,036 1 3,843 20,181 (27,061 ) 20,368 Non-controlling interests — — — — 16 — 16 Total equity 20,368 3,036 1 3,843 20,197 (27,061 ) 20,384 1. Consistent with the presentation of the Group balance sheet we have re-presented commodity derivatives from current and non-current receivables and payables to derivative financial assets and liabilities. 34. Additional disclosures in respect of guaranteed securities continued Cash flow statements Parent Issuer of notes Subsidiary guarantor National Grid plc £m Niagara Mohawk Power Corporation £m British Transco Finance Inc. £m National Grid Gas plc £m Other subsidiaries £m Consolidation adjustments £m National Grid consolidated £m Year ended 31 March 2018 Net cash flow from operating activities – continuing operations 35 662 — 888 3,125 — 4,710 Net cash flow from operating activities – discontinued operations — — — (98 ) (109 ) — (207 ) Net cash flow from/(used in) investing activities – continuing operations 4,660 (473 ) 15 656 (1,759 ) (862 ) 2,237 Net cash flow from/(used in) investing activities – discontinued operations — — — — — — — Net cash flow (used in)/from financing activities – continuing operations (5,785 ) (189 ) (15 ) (1,041 ) (1,148 ) 862 (7,316 ) Net cash flow (used in)/from financing activities – discontinued operations — — — (125 ) (106 ) — (231 ) Net increase/(decrease) in cash and cash equivalents in the year (1,090 ) — — 280 3 — (807 ) Year ended 31 March 2017 Net cash flow from operating activities – continuing operations 53 757 — 918 2,592 — 4,320 Net cash flow from operating activities – discontinued operations — — — 450 459 — 909 Net cash flow from/(used in) investing activities – continuing operations 4,181 (469 ) 15 215 (1,118 ) (6,458 ) (3,634 ) Net cash flow from/(used in) investing activities – discontinued operations — — — 5,618 (6,298 ) — (680 ) Net cash flow (used in)/from financing activities – continuing operations (3,146 ) (288 ) (15 ) (8,322 ) 3,771 6,458 (1,542 ) Net cash flow (used in)/from financing activities – discontinued operations — — — 1,120 491 — 1,611 Net increase/(decrease) in cash and cash equivalents in the year 1,088 — — (1 ) (103 ) — 984 Year ended 31 March 2016 Net cash flow from operating activities – continuing operations 57 580 — 599 3,056 — 4,292 Net cash flow from operating activities – discontinued operations — — — 1,144 (68 ) — 1,076 Net cash flow from/(used in) investing activities – continuing operations 502 (440 ) 13 56 (1,721 ) (1,869 ) (3,459 ) Net cash flow from/(used in) investing activities – discontinued operations — — — (562 ) (15 ) — (577 ) Net cash flow (used in)/from financing activities – continuing operations (555 ) (148 ) (13 ) (1,185 ) (1,173 ) 1,869 (1,205 ) Net cash flow (used in)/from financing activities – discontinued operations — — — (63 ) (60 ) — (123 ) Net (decrease)/increase in cash and cash equivalents in the year 4 (8 ) — (11 ) 19 — 4 Cash dividends were received by National Grid plc from subsidiary undertakings amounting to £950 million during the year ended 31 March 2018 ( 2017 : £6,006 million ; 2016 : £930 million ). Maturity analysis of parent Company borrowings 2018 2017 £m £m Total borrowings are repayable as follows: Less than 1 year 781 1,120 In 1 to 2 years 438 515 In 2 to 3 years — 425 In 3 to 4 years 335 — In 4 to 5 years — 322 More than 5 years — — 1,554 2,382

Post balance sheet events

Post balance sheet events	12 Months Ended
	Mar. 31, 2018
Events After Reporting Period [Abstract]
Post balance sheet events	Post balance sheet events As set out in note 15, on 1 May 2018, the Group announced that it had entered into an agreement with Quadgas Investments BidCo Limited regarding the potential sale of its remaining 25% equity interest in Quadgas HoldCo Limited, the holding company for Cadent Gas Limited. Refer to notes 4 and 15 for details on the accounting implications on the results for the year ended 31 March 2018 in relation to this agreement.

Basis of preparation and rece55

Basis of preparation and recent accounting developments (Policies)	12 Months Ended
	Mar. 31, 2018
Accounting Policies, Changes In Accounting Estimates And Errors [Abstract]
Description of accounting policy for basis of preparation	These consolidated financial statements have been prepared in accordance with International Accounting Standards (IAS) and International Financial Reporting Standards (IFRS) and related interpretations as issued by the IASB and IFRS as adopted by the EU. They are prepared on the basis of all IFRS accounting standards and interpretations that are mandatory for periods ended 31 March 2018 and in accordance with the Companies Act 2006 applicable to companies reporting under IFRS and Article 4 of the EU IAS Regulation. The comparative financial information has also been prepared on this basis. The consolidated financial statements have been prepared on a historical cost basis, except for the recording of pension assets and liabilities, the revaluation of derivative financial instruments and certain commodity contracts and investments classified as available-for-sale. These consolidated financial statements are presented in pounds sterling, which is also the functional currency of the Company. The notes to the financial statements have been prepared on a continuing basis unless otherwise stated. Our income statement and segmental analysis separately identify financial results before and after exceptional items and remeasurements. The Directors believe that presentation of the results in this way is relevant to an understanding of the Group’s financial performance. Presenting financial results before exceptional items and remeasurements is consistent with the way that financial performance is measured by management and reported to the Board and Executive Committee and aids the comparability of reported financial performance from year to year in this context. Further, this year we have adopted a columnar presentation as we consider it improves the clarity of the presentation, and is consistent with the way that financial performance is measured by management and reported to the Board and Executive Committee, and better enables users of the financial statements to understand the results. The inclusion of total profit for the period from continuing operations before exceptional items and remeasurements forms part of the incentive target set annually for remunerating certain Executive Directors. Accordingly we believe it is important for users of the financial statements to understand how this compares to our results on a statutory basis and year-on-year. The preparation of financial statements requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosures of contingent assets and liabilities and the reported amounts of revenue and expenses during the reporting period (see accounting policy E).
Going concern	The Directors considered it appropriate to prepare the financial statements on a going concern basis, having considered the Company’s cash flow forecasts with respect to business planning and treasury management activities. The going concern basis presumes that the Group has adequate resources to remain in operation, and that the Directors intend it to do so, for at least one year from the date the financial statements are signed.
Basis of consolidation	The consolidated financial statements incorporate the results, assets and liabilities of the Company and its subsidiaries, together with a share of the results, assets and liabilities of joint operations. The Group accounts for joint ventures and associates using the equity method of accounting, where the investment is carried at cost plus post-acquisition changes in the share of net assets of the joint venture or associate, less any provision for impairment. A subsidiary is defined as an entity controlled by the Group. Control is achieved where the Group is exposed to, or has the rights to, variable returns from its involvement with the entity and has the power to affect those returns through its power over the entity. Losses in excess of the consolidated interest in joint ventures and associates are not recognised, except where the Company or its subsidiaries have made a commitment to make good those losses. Where necessary, adjustments are made to bring the accounting policies used in the individual financial statements of the Company, subsidiaries, joint operations, joint ventures and associates into line with those used by the Group in its consolidated financial statements under IFRS. Intercompany transactions are eliminated. The results of subsidiaries (other than relating to UK Gas Distribution as described in C below), joint operations, joint ventures and associates acquired or disposed of during the year are included in the consolidated income statement from the effective date of acquisition or up to the effective date of disposal, as appropriate. Acquisitions are accounted for using the acquisition method, where the purchase price is allocated to the identifiable assets acquired and liabilities assumed on a fair value basis and the remainder recognised as goodwill.
Foreign currencies	Transactions in currencies other than the functional currency of the Company or subsidiary concerned are recorded at the rates of exchange prevailing on the dates of the transactions. At each reporting date, monetary assets and liabilities that are denominated in foreign currencies are retranslated at closing exchange rates. Non-monetary assets are not retranslated unless they are carried at fair value. Gains and losses arising on the retranslation of monetary assets and liabilities are included in the income statement, except where the application of hedge accounting requires inclusion in other comprehensive income see note 16. On consolidation, the assets and liabilities of operations that have a functional currency different from the Company’s functional currency of pounds sterling, principally our US operations that have a functional currency of US dollars, are translated at exchange rates prevailing at the reporting date. Income and expense items are translated at the average exchange rates for the period where these do not differ materially from rates at the date of the transaction. Exchange differences arising are recognised in other comprehensive income and transferred to the consolidated translation reserve within other equity reserves see note 26.
Areas of judgement and key sources of estimation uncertainty	The preparation of financial statements requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosures of contingent assets and liabilities, and the reported amounts of revenue and expenses during the reporting period. Actual results could differ from these estimates. Information about such judgements and estimations is contained in the notes to the financial statements, and the key areas are summarised below. Areas of judgement that have the most significant effect on the amounts recognised in the financial statements are as follows: • categorisation of certain items as exceptional items and the definition of adjusted earnings see notes 4 and 7. Key sources of estimation uncertainty that have a significant risk of causing a material adjustment to the carrying amounts of assets and liabilities within the next financial year are as follows: • valuation of liabilities for pensions and other post-retirement benefits see note 23; and • the discount rate and cash flows applied in determining the environmental provisions see note 24. In order to illustrate the impact that changes in assumptions could have on our results and financial position, we have included sensitivity analyses in note 33.
New IFRS accounting standards effective and New IFRS accounting standards and interpretations not yet adopted	FRS 15 ‘Revenue from Contracts with Customers’ IFRS 15 ‘Revenue from Contracts with Customers’ is effective for National Grid for the year ending 31 March 2019. The new standard provides enhanced detail and a five-step revenue recognition approach to reflect the transfer of goods and services to customers. The core principle of IFRS 15 is that an entity recognises revenue related to the transfer of promised goods or services when control of the goods or services passes to customers. The amount of revenue recognised should reflect the consideration to which the entity expects to be entitled in exchange for those goods or services. This differs from the principle under the current revenue standard that requires an assessment of when risks and rewards of goods and services are transferred rather than control of those goods or services. Detailed reviews of revenue arrangements in the UK and US have been undertaken prior to our transition to IFRS 15 on 1 April 2018. We will adopt the modified retrospective approach whereby the historical cumulative transition adjustment is reflected through retained earnings. There are two types of revenue arrangements that will be impacted on transition to IFRS 15. The financial impacts and the transition adjustment to retained earnings are described further: • There are certain pass-through revenues (principally revenues collected on behalf of the Scottish and Offshore transmission operators) where the principal/agency assessment changes on transition to IFRS 15. In moving from a risk and reward model to a control model, we will no longer record our revenues collected on behalf of the Scottish and Offshore transmission operators as principal as we do not control the Scottish or Offshore transmission networks. If we had adopted IFRS 15 in 2017/18, both revenues and operating costs would have been £1,056 million lower, with no impact to profit as a result of this change. There will be no transition adjustment as a result of this change; and • Across our subsidiaries in the UK and the US, our customers provide contributions for certain capital works (e.g. connections) which we continue to own on completion of the works. In our electricity business in the UK, we currently recognise customer contributions for connections over time as we have an ongoing contractual condition to maintain connections over their lives. In our UK Gas Transmission business, we recognise customer contributions when the connection is completed (the contractual conditions of the connection agreement do not explicitly require connections to be maintained over the life of the connection). In the US, revenue is also recognised when the connection is completed. Under IFRS 15, connection contributions in our subsidiaries will be deferred and released into the income statement as revenue over the life of the network. We have reached this conclusion because our customers cannot benefit from a connection without the use of our utility network; access to our network through the connection is satisfied over time. In the UK, we also have arrangements where our customers make contributions for diversions. These are currently deferred over the life of our network. Under IFRS 15, these revenues are recognised on completion of the diversion as there are no ongoing performance obligations to satisfy. Had we adopted IFRS 15 in 2017/18, revenues would have been approximately £83 million lower, as revenues from connections in the US and in UK Gas Transmission that were recognised up-front would have been deferred over the life of the network. The decrease in profit after tax in our subsidiaries would have been £56 million . The transition adjustment through retained earnings of £167 million will result in an increase to deferred revenues of approximately £240 million and a corresponding deferred tax impact of £73 million . iii) IFRS 16 ‘Leases’ IFRS 16 ‘Leases’ is effective for National Grid for the year ending 31 March 2020. The Group enters into a significant number of operating lease transactions. Under IFRS 16, our operating leases will be accounted for on the balance sheet as ‘right-of-use’ assets. This treatment will increase both our assets and liabilities and subsequently, will result in an increase in finance costs and depreciation and a reduction in operating costs. The outcome of our conclusions will have an impact on how we account for our operating leases. We are also performing an assessment of our revenue, service contracts and power purchase contracts to determine whether we have the right to use assets under those contracts and whether they fall within the scope of IFRS 16. We plan to apply IFRS 16 using the modified retrospective approach, whereby comparatives will not be restated on adoption of the new standard but instead a cumulative adjustment will be reflected in retained earnings. iv) Other In addition, the following new accounting standards and amendments to existing standards have been issued but are not yet effective or have not yet been endorsed by the EU: • Amendments to IFRS 2 ‘Share-based payment’; • IFRIC 22 ‘Foreign Currency Transactions and Advance Consideration’; • IFRIC 23 ‘Uncertainty over Income Tax Treatments’; • Amendments to IAS 40 ‘Investment Property’; • Amendments to IAS 28: 'Investments in associates' – Long-term interests in associates and joint ventures; • Annual Improvements to IFRS Standards 2015-2017 Cycle; • IFRS 17 ‘Insurance Contracts’; and • Amendments to IAS 19 ‘Employee Benefits’. Effective dates remain subject to the EU endorsement process. The Group enters into a significant number of transactions that fall within the scope of IFRS 9 ‘Financial Instruments’, IFRS 15 ‘Revenue from Contracts with Customers’ and IFRS 16 ‘Leases’. We are assessing the likely impact of these standards on the Group’s financial statements. i) IFRS 9 ‘Financial Instruments’ IFRS 9 ‘Financial Instruments’ is effective for National Grid for the year ending 31 March 2019. The change to IFRS 9 principally impacts the accounting for the classification and measurement of financial instruments, impairment of financial assets and hedge accounting. The Group has elected not to restate comparatives on initial application of IFRS 9. The full impact of adopting IFRS 9 will depend on the financial instruments that the Group has during the year ending 31 March 2019 as well as on economic conditions and judgements made as at the year end. The Group has performed an assessment of the potential impact of adopting IFRS 9 based on the financial instruments and hedging relationships as at the date of adoption of IFRS 9 (1 April 2018). It is not expected that the adoption of IFRS 9 will materially impact our profits or net assets on transition or prospectively. Classification and measurement: financial assets The number of categories of financial assets has been reduced under IFRS 9 compared to IAS 39. Under IFRS 9 the classification of financial assets is based on the business model within which the asset is held and the contractual terms of the asset. There are three principal classification categories for financial assets that are debt instruments: (i) amortised cost, (ii) fair value through other comprehensive income (FVOCI), and (iii) fair value through profit or loss (FVTPL). Equity investments are either classified as (i) FVTPL or (ii) FVOCI. Where assets or liabilities are measured at FVTPL, any fair value movements will be reported as remeasurements. If measured at FVOCI, realised gains on equity investments are not recycled to the income statement but instead are transferred directly to retained profits. The largest reallocation of financial assets will be from available-for-sale investments to FVTPL and relates to over £2 billion Group investments in money market funds and insurance company investments in mutual funds, where the contractual terms are such that they do not qualify for any other category. All other available-for-sale investments will be categorised as FVOCI under IFRS 9. The change to the asset classification rules will have no impact on reported Group net assets, although there will be some changes to reserves at transition. Classification and measurement: financial liabilities Under IFRS 9 financial liabilities can be designated at FVTPL to eliminate an accounting mismatch. Where financial liabilities are designated at FVTPL, changes in their fair value due to credit risk are presented in other comprehensive income. Remaining changes in fair value are presented in the income statement. With effect from 1 April 2018 the Group will take the fair value option for one issued zero coupon liability to reduce the measurement mismatch against some derivatives. This change will result in a reduction of reported net assets of less than £50 million . Impairment The impairment model under IFRS 9 reflects expected credit losses, as opposed to only incurred losses under IAS 39. The new impairment model will apply to the Group’s financial assets that are debt instruments measured at amortised cost or FVOCI as well as the Group’s trade receivables. The Group expects to apply the simplified approach, recognising lifetime expected losses for its trade receivables. The Group’s preliminary calculation of the loss allowance for these assets as at 31 March 2018 results in an immaterial impact compared to under IAS 39. The Group’s other investments in debt instruments that are subject to the IFRS 9 impairment model are determined to be low credit risk at 31 March 2018. The Group intends to apply the low credit risk simplification in IFRS 9, which allows the Group to assume that there has not been a significant increase in credit risk since initial recognition of these assets, and therefore recognise a loss allowance for only 12 -month expected credit losses as at 1 April 2018. The adjustment to the opening reserves in respect of this is not expected to be significant. The Group has adopted the following amendments to standards: • annual improvements to IFRSs 2014-2016 Cycle; • amendments to IAS 7 ‘Statement of cash flows’; and • amendments to IAS 12 ‘Income taxes’. The adoption of these amendments has had no material impact on the Group’s results or financial statement disclosures.
Hedge accounting	On initial application of IFRS 9, an entity may choose to continue to apply the hedge accounting requirements of IAS 39 instead of those of IFRS 9. The Group has elected to apply the IFRS 9 hedge accounting requirements because they more closely align with the Group’s risk management policies. An assessment of the Group’s designated hedging relationships under IAS 39 has been performed and it has been determined that all would qualify as continuing hedge relationships under IFRS 9. However, in order to apply elective changes to the treatment of costs of hedging, certain relationships will be formally redesignated from the date of adoption. The Group is considering additional opportunities to apply hedge accounting under IFRS 9. The Group does not anticipate the application of IFRS 9 hedge accounting requirements will have a material impact on the Group’s consolidated financial statements.
Segmental analysis	This note sets out the financial performance for the year split into the different parts of the business (operating segments). The performance of these operating segments is monitored and managed on a day-to-day basis. Revenue and the results of the business are analysed by operating segment, based on the information the Board of Directors uses internally for the purposes of evaluating the performance of operating segments and determining resource allocation between operating segments. The Board is National Grid’s chief operating decision maker (as defined by IFRS 8 ‘Operating Segments’) and assesses the profitability of operations principally on the basis of operating profit before exceptional items and remeasurements (see note 4). As a matter of course, the Board also considers profitability by segment, excluding the effect of timing. However, the measure of profit disclosed in this note is operating profit before exceptional items and remeasurements as this is the measure that is most consistent with the IFRS results reported within these financial statements. Our strategy in action The Group owns a portfolio of businesses that range from businesses with high levels of investment and growth to cash generative developed assets with lower investment requirements (such as National Grid Metering, included within NGV and Other). The majority of revenue is generated from regulated operating segments in the UK and US. The Group works with its regulators to obtain agreements that balance the risks faced with the opportunity to deliver reasonable returns for investors. When investing in NGV and Other, the Group aims to leverage its core capabilities to deliver higher returns for investors. The regulated businesses earn revenue for the transmission, distribution and generation services they have provided during the year. In any one year, the revenue recognised may differ from that allowed under the Group’s regulatory agreements and any such timing differences are adjusted through future prices. NGV and Other businesses earn revenue in line with their contractual terms.
Operating costs	Below we have presented separately certain items included in our operating costs from continuing operations. These include a breakdown of payroll costs (including disclosure of amounts paid to key management personnel) and fees paid to our auditors.
Exceptional items and remeasurements	To monitor our financial performance, we use a profit measure that excludes certain income and expenses. We call that measure ‘business performance’ or ‘adjusted profit’. We exclude items from business performance because, if included, these items could distort understanding of our performance for the year and the comparability between periods. This note analyses these items, which are included in our results for the year but are excluded from business performance.
Finance income and costs	This note details the interest income generated by our financial assets and interest expense incurred on our financial liabilities. It also includes the net interest on our pensions and other post-retirement assets. In reporting business performance, we adjust net financing costs to exclude any net gains or losses on derivative financial instruments included in remeasurements. In addition, significant debt redemption costs are typically treated as exceptional (see note 4).
Tax	Tax is payable in the territories where we operate, mainly the UK and the US. This note gives further details of the total tax charge and tax liabilities, including current and deferred tax. The current tax charge is the tax payable on this year’s taxable profits. Deferred tax is an accounting adjustment to provide for tax that is expected to arise in the future due to differences in the accounting and tax bases.
Earnings per share (EPS)	EPS is the amount of post-tax profit attributable to each ordinary share. Basic EPS is calculated on profit for the year attributable to equity shareholders divided by the weighted average number of shares in issue during the year. Diluted EPS shows what the impact would be if all outstanding share options were exercised and treated as ordinary shares at year end. The weighted average number of shares is increased by additional shares issued as scrip dividends and reduced by shares repurchased by the Company during the year. The earnings per share calculations are based on profit after tax attributable to equity shareholders of the Company which excludes non-controlling interests.
Dividends	Dividends represent the return of profits to shareholders. Dividends are paid as an amount per ordinary share held. We retain part of the profits generated in the year to meet future growth plans and pay out the remainder in accordance with our dividend policy.
Discontinued operations	As described further in note 9, on 8 December 2016 the Group entered into a sale and purchase agreement to dispose of a 61% controlling stake in the UK Gas Distribution business. The disposal completed on 31 March 2017 and the Group has retained a 39% interest in the business. As a result, all assets and liabilities of UK Gas Distribution were deemed to be disposed of and a 39% interest reacquired. The 39% retained interest is classified as an associate on the basis that the Group retains significant influence over the business through its retained stake. The Group has the ability to appoint 4 of the 12 directors on the board of Quadgas HoldCo Limited. In addition, the Group entered into a Further Acquisition Agreement (FAA) over a further 14% interest. Refer to note 15 for further details. The Group classified UK Gas Distribution as held for sale as of 8 December 2016, when it became highly probable that the value of the business to the Group would be recovered through sale rather than continuing ownership. As UK Gas Distribution represents a separate major line of business, the business was classified as a discontinued operation in the 2016/17 consolidated income statement. This continues to be reflected in the consolidated income statement and the consolidated statement of comprehensive income, as well as earnings per share (EPS) split between continuing and discontinued operations. Our results and cash flows of significant assets or businesses sold during the year are shown separately from our continuing operations. Assets and businesses are classified as held for sale when their carrying amounts are recovered through sale rather than through continuing use. It only meets the held for sale condition when the assets are ready for immediate sale in their present condition, management is committed to the sale and it is highly probable that the sale will complete within one year. Depreciation ceases on assets and businesses when they are classified as held for sale and the assets and businesses are impaired if the proceeds less sale costs fall short of the carrying value. As a result of the sale of a 61% controlling interest in UK Gas Distribution on 31 March 2017, we are required to report our earnings for the Group excluding UK Gas Distribution (‘continuing operations’) separately from the results of that business, which we report within ‘discontinued operations’. The gain recognised by the Group on sale is analysed in the detail of the note below. All costs associated with the transaction, including those associated with separation and setting up UK Gas Distribution are shown as a deduction from the proceeds received. Any adjustments arising as part of the completion adjustments finalised within the measurement period would result in a further gain or loss on disposal to be reported within discontinued operations in the current period.
Goodwill	Goodwill represents the excess of what we paid to acquire businesses over the fair value of their net assets at the acquisition date. We assess whether goodwill is recoverable each year by performing an impairment review.
Other intangible assets	Other intangible assets include software which is written down (amortised) over the length of period we expect to receive a benefit from the asset.
Property, plant and equipment	The following note shows the physical assets controlled by us. The cost of these assets primarily represents the amount initially paid for them. This includes both their purchase price and the construction and other costs associated with getting them ready for operation. A depreciation expense is charged to the income statement to reflect annual wear and tear and the reduced value of the asset over time. Depreciation is calculated by estimating the number of years we expect the asset to be used (useful economic life) and charging the cost of the asset to the income statement equally over this period. Our strategy in action We operate an energy networks business and therefore have a significant physical asset base. We continue to invest in our networks to maintain reliability, create new customer connections and ensure our networks are flexible and resilient. Our business plan envisages these additional investments will be funded through a mixture of cash generated from operations and the issue of new debt.
Other non-current assets	Other non-current assets include assets that do not fall into any other non-current asset category (such as goodwill or property, plant and equipment) where the benefit to be received from the asset is not due to be received until after 31 March 2019.
Financial and other investments	Financial and other investments include three main categories. Assets classified as available-for-sale typically represent investments in short-term money funds and quoted investments in equities or bonds of other companies. The second category comprises long-term loans to our associates and joint ventures. The third category is other loans and receivables which includes bank deposits with a maturity of greater than three months, and cash balances that cannot be readily used in operations, principally collateral pledged against derivative holdings.
Investments in joint ventures and associates	Investments in joint ventures and associates represent businesses we do not control, but instead exercise joint control or significant influence. While we present consolidated results in these financial statements as if we were one company, our legal structure is such that there are a number of different operating and holding companies that contribute to the overall result. This structure has evolved through acquisitions as well as regulatory requirements to have certain activities within separate legal entities.
Derivative financial instruments	Derivatives are financial instruments that derive their value from the price of an underlying item such as interest rates, foreign exchange rates, credit spreads, commodities, equity or other indices. In accordance with Board approved policies, derivatives are transacted generally to manage our exposure to fluctuations in interest rate, foreign exchange and our operational market risks from our commodity activities. Our derivatives are split into the broad categories analysed below: • derivatives managing risks to interest rate and foreign exchange rate. Specifically we use these derivatives to manage risks from the financing portfolio, to optimise the overall cost of accessing the debt capital markets, managing the exposure to holdings in foreign operations and other contractual operational cash flows; and • derivatives managing our price and supply risks from our commodity activity.
Inventories and current intangible assets	Inventories represent assets that we intend to use in order to generate revenue in the short-term, either by selling the asset itself (for example, fuel stocks) or by using it to fulfil a service to a customer or to maintain our network (consumables).
Trade and other receivables	Trade and other receivables are amounts which are due from our customers for services we have provided. Other receivables also include prepayments made by us, for example, property lease rentals paid in advance.
Cash and cash equivalents	Cash and cash equivalents include cash balances, together with short-term investments with an original maturity of less than three months that are readily convertible to cash.
Borrowings and Borrowing facilities	We borrow money primarily in the form of bonds and bank loans. These are for a fixed term and may have fixed or floating interest rates or are linked to RPI. As indicated in note 16, we use derivatives to manage risks associated with interest rates and foreign exchange. Further details on our net debt can be found in note 27. Our strategy in action Our price controls and rate plans require us to fund our networks within a certain ratio of debt to equity and, as a result, we have issued a significant amount of debt. As we continue to invest in our networks, the value of debt is expected to increase over time. To maintain a strong balance sheet and to allow us to access capital markets at commercially acceptable interest rates, we balance the amount of debt we issue with the value of our assets, and take account of certain other metrics used by credit rating agencies. To support our long-term liquidity requirements and provide backup to commercial paper and other borrowings, we agree loan facilities with financial institutions over and above the value of borrowings that may be required. These facilities have never been drawn, and our undrawn amounts are listed below.
Trade and other payables	Trade and other payables include amounts owed to suppliers, tax authorities and other parties which are due to be settled within 12 months. The total also includes deferred income, which represents monies received from customers but for which we have not yet delivered the associated service. These amounts are recognised as revenue when the service is provided.
Other non-current liabilities	Other non-current liabilities include deferred income which will not be recognised as income until after 31 March 2019. It also includes payables that are not due until after that date.
Pensions and other post-retirement benefits	The majority of employees are either members of a DB (defined benefit) or a DC (defined contribution) pension scheme. In the US we also provide healthcare and life insurance benefits to eligible retired US employees. The fair value of associated scheme assets and present value of DB obligations are updated annually in accordance with IAS 19 (revised). We separately present our UK and US pension schemes to show geographical split. Below we provide a more detailed analysis of the amounts recorded in the primary financial statements and the actuarial assumptions used to value the DB obligations.
Provisions	We make provisions when an obligation exists, resulting from a past event and it is probable that cash will be paid to settle it, but the exact amount of cash required can only be estimated. The main estimates relate to environmental remediation and decommissioning costs for various sites we own or have owned and other provisions, including restructuring plans and lease contracts we have entered into that are now loss making. The evaluation of the likelihood of the contingent events has required best judgement by management regarding the probability of exposure to potential loss. Should circumstances change following unforeseeable developments, the likelihood could alter. Our strategy in action We are committed to the protection and enhancement of the environment. However, we have acquired, owned and operated a number of businesses which have, during the course of their operations, created an environmental impact. Therefore we have a provision that reflects the expected cost to remediate these sites. Current operations will seldom result in new sites with significant expected costs being added to the provision.
Share capital	Ordinary share capital represents the total number of shares issued which are publicly traded. We also disclose the number of treasury shares the Company holds, which are shares that the Company has bought itself, predominately to actively manage scrip issuances and employee share option plan liabilities.
Other equity reserves	Other equity reserves are different categories of equity as required by accounting standards and represent the impact of a number of our historical transactions.
Net debt	Net debt represents the amount of borrowings and overdrafts less cash, financial investments and related financing derivatives.
Commitments and contingencies	Commitments are those amounts that we are contractually required to pay in the future as long as the other party meets its obligations. These commitments primarily relate to operating lease rentals, energy purchase agreements and contracts for the purchase of assets which, in many cases, extend over a long period of time. We also disclose any contingencies, which include guarantees that companies have given, where we pledge assets against current obligations that will remain for a specific period.
Related party transactions	Related parties include joint ventures, associates, investments and key management personnel.
Financial risk management	Our activities expose us to a variety of financial risks including currency risk, interest rate risk, commodity price risk, credit risk, capital risk and liquidity risk. Our risk management programme focuses on the unpredictability of financial markets and seeks to minimise potential volatility of financial performance from these risks. We use financial instruments, including derivative financial instruments, to manage risks of this type. This note describes our approach to managing risk, including an analysis of assets and liabilities by currency type and an analysis of interest rate category for our net debt. We are required by accounting standards to also include a number of specific disclosures (such as a maturity analysis of contractual undiscounted cash flows) and have included these requirements below.
Sensitivities	In order to give a clearer picture of the impact on our results or financial position of potential changes in significant estimates and assumptions, the following sensitivities are presented. These sensitivities are hypothetical, as they are based on assumptions and conditions prevailing at the year-end, and should be used with caution. The effects provided are not necessarily indicative of the actual effects that would be experienced because our actual exposures are constantly changing.
Additional disclosures in respect of guaranteed securities	We have three debt issuances (including preferred shares) that are listed on a US national securities exchange and are guaranteed by other companies in the Group. These guarantors commit to honour any liabilities should the company issuing the debt have any financial difficulties. In order to provide debt holders with information on the financial stability of the companies providing the guarantees, we are required to disclose individual financial information for these companies. We have chosen to include this information in the Group financial statements rather than submitting separate stand-alone financial statements.

Segmental analysis (Tables)

Segmental analysis (Tables)	3 Months Ended	12 Months Ended
	Jun. 30, 2018	Mar. 31, 2018
Operating Segments [Abstract]
Schedule of operating segments	The following table describes the main activities for each reportable operating segment: UK Electricity Transmission High-voltage electricity transmission networks in England and Wales UK Gas Transmission High-pressure gas transmission networks in Great Britain and LNG storage activities US Regulated Gas distribution networks, electricity distribution networks and high-voltage electricity transmission networks in New York and New England and electricity generation facilities in New York	(a) Revenue 2018 2017 2016 Total £m Sales between segments £m Sales to third parties £m Total Sales Sales Total Sales Sales Operating segments – continuing operations: UK Electricity Transmission 4,154 (28 ) 4,126 4,439 (29) 4,410 3,977 (20) 3,957 UK Gas Transmission 1,091 (9 ) 1,082 1,080 (99) 981 1,047 (109) 938 US Regulated 9,272 — 9,272 8,931 — 8,931 7,493 — 7,493 NGV and Other 1 776 (6 ) 770 713 — 713 824 — 824 Total revenue from continuing operations 15,293 (43 ) 15,250 15,163 (128) 15,035 13,341 (129) 13,212 Split by geographical areas – continuing operations: UK 5,938 6,064 5,619 US 9,312 8,971 7,593 15,250 15,035 13,212 1. Included within NGV and Other is £593 million ( 2017 : £604 million ; 2016 : £719 million ) of revenue relating to NGV. (b) Operating profit A reconciliation of the operating segments’ measure of profit to profit before tax from continuing operations is provided below. Further details of the exceptional items and remeasurements are provided in note 4. Before exceptional items After exceptional items 2018 2017 2016 2018 2017 2016 £m £m £m £m £m £m Operating segments – continuing operations: UK Electricity Transmission 1,041 1,372 1,173 1,041 1,361 1,173 UK Gas Transmission 487 511 486 487 507 486 US Regulated 1,698 1,713 1,185 1,734 1,278 1,196 NGV and Other 1 231 177 370 231 62 370 Total operating profit from continuing operations 3,457 3,773 3,214 3,493 3,208 3,225 Split by geographical area – continuing operations: UK 1,840 2,118 2,007 1,840 1,988 2,007 US 1,617 1,655 1,207 1,653 1,220 1,218 3,457 3,773 3,214 3,493 3,208 3,225 Below we reconcile total operating profit from continuing operations to profit before tax from continuing operations. We have shown the share of post-tax results of joint ventures and associates disaggregated between those held within NGV and Other and our retained 39% interest in the UK Gas Distribution business (Cadent 2 ). Operating exceptional items and remeasurements of £nil ( 2017 : £11 million cost; 2016 : £nil ) detailed in note 4 are attributable to UK Electricity Transmission; £nil ( 2017 : £4 million cost; 2016 : £nil ) to UK Gas Transmission; £36 million gain ( 2017 : £435 million cost; 2016 : £11 million gain) to US Regulated; and £nil ( 2017 : £115 million cost; 2016 : £nil ) to NGV and Other. Reconciliation to profit before tax: Operating profit from continuing operations 3,457 3,773 3,214 3,493 3,208 3,225 Finance income 154 53 22 154 53 22 Finance costs (1,128 ) (1,082 ) (878 ) (899 ) (1,140 ) (977 ) Share of post-tax results of joint ventures and associates: Cadent 2 123 — — (89 ) — — NGV and Other 44 63 59 49 63 59 Profit before tax from continuing operations 2,650 2,807 2,417 2,708 2,184 2,329 1. Included within NGV and Other is £234 million ( 2017 : £239 million ; 2016 : £394 million ) of operating profit (both before and after exceptional items and remeasurements) relating to NGV. 2. Investment held through Quadgas HoldCo Limited. 2. Segmental analysis continued (c) Capital expenditure Net book value of property, plant and Capital expenditure 1 Depreciation and amortisation 2018 2017 2016 2018 2017 2016 2018 2017 2016 £m £m £m £m £m £m £m £m £m Operating segments: UK Electricity Transmission 13,028 12,515 11,907 999 1,027 1,084 (475 ) (421 ) (390 ) UK Gas Transmission 4,280 4,165 4,140 310 214 186 (194 ) (186 ) (178 ) US Regulated 20,953 21,638 17,490 2,424 2,247 1,856 (635 ) (642 ) (535 ) NGV and Other 2 2,491 2,430 2,291 341 247 201 (226 ) (232 ) (208 ) Total from continuing operations 40,752 40,748 35,828 4,074 3,735 3,327 (1,530 ) (1,481 ) (1,311 ) Split by geographical area – continuing operations: UK 18,772 18,102 17,491 1,527 1,357 1,386 (804 ) (753 ) (715 ) US 21,980 22,646 18,337 2,547 2,378 1,941 (726 ) (728 ) (596 ) 40,752 40,748 35,828 4,074 3,735 3,327 (1,530 ) (1,481 ) (1,311 ) Asset type: Property, plant and equipment 39,853 39,825 35,074 3,901 3,507 3,130 (1,392 ) (1,348 ) (1,207 ) Non-current intangible assets 899 923 754 173 228 197 (138 ) (133 ) (104 ) Total from continuing operations 40,752 40,748 35,828 4,074 3,735 3,327 (1,530 ) (1,481 ) (1,311 ) 1. Represents additions to property, plant and equipment and non-current intangibles but excludes additional investments in and loans to joint ventures and associates. 2. Included within NGV and Other are assets with a net book value of £1,454 million ( 2017 : £1,432 million ; 2016 : £1,482 million ), capital expenditure of £186 million ( 2017 : £98 million ; 2016 : £93 million ) and depreciation and amortisation of £143 million ( 2017 : £143 million ; 2016 : £144 million ) relating to NGV.

Operating costs (Tables)

Operating costs (Tables)	12 Months Ended
	Mar. 31, 2018
Analysis of income and expense [abstract]
Schedule of expenses by nature	Before exceptional items and remeasurements Exceptional items and remeasurements Total 2018 2017 2016 2018 2017 2016 2018 2017 2016 £m £m £m £m £m £m £m £m £m Depreciation and amortisation 1,530 1,481 1,311 — — — 1,530 1,481 1,311 Payroll costs 1,648 1,578 1,337 — — — 1,648 1,578 1,337 Purchases of electricity 1,299 1,143 1,304 (14 ) (46 ) 8 1,285 1,097 1,312 Purchases of gas 1,539 1,241 986 4 (22 ) (19 ) 1,543 1,219 967 Rates and property taxes 1,057 1,042 899 — — — 1,057 1,042 899 Balancing Services Incentive Scheme 1,012 1,120 907 — — — 1,012 1,120 907 Payments to other UK network owners 1,043 1,008 971 — — — 1,043 1,008 971 Other 2,665 2,649 2,283 (26 ) 633 — 2,639 3,282 2,283 11,793 11,262 9,998 (36 ) 565 (11 ) 11,757 11,827 9,987 Operating costs include: Inventory consumed 367 296 274 Operating leases 102 98 91 Research and development expenditure 13 14 19 (a) Payroll costs 2018 2017 2016 £m £m £m Wages and salaries 1 1,998 1,852 1,553 Social security costs 157 145 120 Defined contribution scheme costs 65 58 47 Defined benefit pension costs (see note 23) 156 151 154 Share-based payments 16 32 21 Severance costs (excluding pension costs) 7 5 4 2,399 2,243 1,899 Less: payroll costs capitalised (751 ) (665 ) (562 ) Total payroll costs 1,648 1,578 1,337 1. Included within wages and salaries are US other post-retirement benefit costs of £46 million ( 2017 : £53 million ; 2016 : £52 million ). For further information refer to note 23.
Schedule of additional information	(b) Number of employees 31 March 2018 Monthly 31 March 2017 Monthly 31 March 2016 Monthly UK 6,517 6,431 6,265 6,291 6,224 6,067 US 16,506 16,274 15,867 15,752 14,830 14,775 Total number of employees 23,023 22,705 22,132 22,043 21,054 20,842 3. Operating costs continued (c) Key management compensation 2018 2017 2016 £m £m £m Short-term employee benefits 8 8 9 Post-employment benefits 1 1 1 Share-based payments 3 6 4 Total key management compensation 12 15 14 Key management compensation relates to the Board, including the Executive Directors and Non-executive Directors for the years presented. (d) Directors’ emoluments Details of Executive Directors’ emoluments are contained in the Remuneration Report on page 70 and those of Non-executive Directors on page 75. (e) Auditors’ remuneration Auditors’ remuneration is presented below in accordance with the requirements of the Companies Act 2006 and the principal accountant fees and services disclosure requirements of Item 16C of Form 20-F: 2018 2017 2016 £m £m £m Audit fees payable to the parent Company’s auditors 1 and their associates in respect of: Audit of the parent Company’s individual and consolidated financial statements 2 2.7 1.5 1.3 The auditing of accounts of any associate of the Company 7.5 13.7 9.2 Other services supplied 3 3.9 4.6 3.6 14.1 19.8 14.1 Total other services 4 Tax fees: Tax compliance services 0.3 0.4 0.5 Tax advisory services — 0.1 — All other fees: Other assurance services 5 0.7 4.6 4.3 Services relating to corporate finance transactions not covered above 6 — 5.9 1.6 Other non-audit services not covered above 7 0.9 6.3 2.5 1.9 17.3 8.9 Total auditors’ remuneration 16.0 37.1 23.0 1. Deloitte LLP became the Group’s principal auditor for the year ended 31 March 2018 . PricewaterhouseCoopers LLP (PwC) was the principal auditor for the years ended 31 March 2017 and 31 March 2016 . 2. Audit fees in each year represent fees for the audit of the Company’s financial statements and regulatory reporting for the years ended 31 March 2018 , 2017 and 2016 . 3. Other services supplied represent fees payable for services in relation to other statutory filings or engagements that are required to be carried out by the auditors. In particular, this includes fees for reports under section 404 of the US Public Company Accounting Reform and Investor Protection Act of 2002 (Sarbanes-Oxley), audit reports on regulatory returns and the review of interim financial statements for the six-month periods ended 30 September 2017, 2016 and 2015 respectively. 4. There were no audit related fees as described in Item 16C(b) of Form 20-F. 5. Principally amounts relating to assurance services provided in relation to comfort letters for debt issuances. In 2016/17, amounts represented assurance services undertaken by PwC in relation to the sale of UK Gas Distribution and data assurance work in respect of financial information included in US rate filings. 6. Vendor due diligence and other transaction services in relation to the sale of UK Gas Distribution. 7. Fees for other non-audit services – projects including services provided to the UK Property business, relating to evaluating possible options for the use of property assets. In 2016/17, services related principally to PwC assisting the Company with separation activities in relation to the sale of UK Gas Distribution.

Exceptional items and remeasu58

Exceptional items and remeasurements (Tables)	12 Months Ended
	Mar. 31, 2018
Exceptional Items And Remeasurements [Abstract]
Schedule of exceptional items and remeasurements from continuing operations	Exceptional items and remeasurements from continuing operations 2018 2017 2016 £m £m £m Included within operating profit Exceptional items: Environmental charges — (526 ) — Gas holder demolition costs — (107 ) — Final settlement of LIPA MSA Transition 26 — — 26 (633 ) — Remeasurements – commodity contract derivatives 10 68 11 36 (565 ) 11 Included within finance costs Remeasurements – net gains/(losses) on derivative financial instruments 229 (58 ) (99 ) Included within share of post-tax results of joint ventures and associates Remeasurements – net gains on financial instruments 1 — — Exceptional items: Deferred tax arising on the reduction in US corporation tax rate 5 — — Impairment of investment in Quadgas HoldCo Limited (213 ) — — (207 ) — — Total included within profit before tax 58 (623 ) (88 ) Included within tax Exceptional items – credits arising on items not included in profit before tax: Deferred tax arising on the reduction in the UK corporation tax rate — 94 162 Deferred tax arising on the reduction in the US corporation tax rate 1,510 — — Tax on exceptional items (9 ) 227 — Tax on remeasurements (28 ) (29 ) 15 1,473 292 177 Total exceptional items and remeasurements after tax 1,531 (331 ) 89 Analysis of total exceptional items and remeasurements after tax Exceptional items after tax 1,319 (312 ) 162 Remeasurements after tax 212 (19 ) (73 ) Total exceptional items and remeasurements after tax 1,531 (331 ) 89

Finance income and costs (Table

Finance income and costs (Tables)	12 Months Ended
	Mar. 31, 2018
Analysis of income and expense [abstract]
Schedule of finance income and costs	2018 2017 2016 Notes £m £m £m Finance income Interest income on financial instruments: Bank deposits and other financial assets 81 28 22 Gains on disposal of available-for-sale investments 73 25 — 154 53 22 Finance costs Net interest on pensions and other post-retirement benefit obligations 23 (65 ) (107 ) (111 ) Interest expense on financial liabilities held at amortised cost: Bank loans and overdrafts (87 ) (59 ) (28 ) Other borrowings (1,030 ) (927 ) (792 ) Derivatives 12 (8 ) 37 Unwinding of discount on provisions 24 (75 ) (73 ) (69 ) Other interest (11 ) (17 ) (27 ) Less: interest capitalised 1 128 109 112 (1,128 ) (1,082 ) (878 ) Remeasurements Net gains/(losses) on derivative financial instruments included in remeasurements 2 : Ineffectiveness on derivatives designated as: Fair value hedges 3 34 33 39 Cash flow hedges 10 (12 ) (15 ) Net investment hedges – undesignated forward rate risk 5 60 (34 ) Derivatives not designated as hedges or ineligible for hedge accounting 4 180 (139 ) (89 ) 229 (58 ) (99 ) (899 ) (1,140 ) (977 ) Net finance costs from continuing operations (745 ) (1,087 ) (955 ) 1. Interest on funding attributable to assets in the course of construction in the current year was capitalised at a rate of 4.1% ( 2017 : 3.4% ; 2016 : 3.3% ). In the UK, capitalised interest qualifies for a current year tax deduction with tax relief claimed of £20 million ( 2017 : £18 million ; 2016 : £19 million ). In the US, capitalised interest is added to the cost of plant and qualifies for tax depreciation allowances. 2. Includes a net foreign exchange loss on financing activities of £314 million ( 2017 : £264 million loss; 2016 : £407 million loss) offset by foreign exchange gains and losses on derivative financial instruments measured at fair value. 3. Includes a net loss on instruments designated as fair value hedges of £90 million ( 2017 : £27 million loss; 2016 : £34 million gain) and a net gain of £124 million ( 2017 : £60 million gain; 2016 : £5 million gain) arising from fair value adjustments to the carrying value of debt. 4. Includes £110 million gain on the Further Acquisition Agreement (FAA) derivative financial instrument relating to the put/call option over a 14% interest in Quadgas HoldCo Limited. Further details can be found in note 15.

Tax (Tables)

Tax (Tables)	12 Months Ended
	Mar. 31, 2018
Income Taxes [Abstract]
Disclosure Of Major Components Of Tax Expense (Income)	Tax (credited)/charged to the income statement – continuing operations 2018 2017 2016 £m £m £m Tax before exceptional items and remeasurements 589 666 604 Exceptional tax on items not included in profit before tax (see note 4) (1,510 ) (94 ) (162 ) Tax on other exceptional items and remeasurements 37 (198 ) (15 ) Tax on total exceptional items and remeasurements (1,473 ) (292 ) (177 ) Total tax (credit)/charge from continuing operations (884 ) 374 427 The tax credit for the year can be analysed as follows: 2018 2017 2016 £m £m £m Current tax: UK corporation tax at 19% (2017: 20%; 2016: 20%) 205 225 239 UK corporation tax adjustment in respect of prior years (18 ) (47 ) (5 ) 187 178 234 Overseas corporation tax 15 — 38 Overseas corporation tax adjustment in respect of prior years (4 ) 1 (19 ) 11 1 19 Total current tax from continuing operations 198 179 253 Deferred tax: UK deferred tax 65 (9 ) (80 ) UK deferred tax adjustment in respect of prior years (2 ) (18 ) 24 63 (27 ) (56 ) Overseas deferred tax (1,155 ) 224 229 Overseas deferred tax adjustment in respect of prior years 10 (2 ) 1 (1,145 ) 222 230 Total deferred tax from continuing operations (1,082 ) 195 174 Total tax (credit)/charge from continuing operations (884 ) 374 427
Disclosure Of Effective Applicable Tax Rate	Tax as a percentage of profit before tax 2018 2017 2016 % % % Before exceptional items and remeasurements – continuing operations 22.2 23.7 25.0 After exceptional items and remeasurements – continuing operations (32.6 ) 17.1 18.3
Disclosure Of Income Tax Relating To Components Of Other Comprehensive Income And Items Credited (Charged) Directly To Equity	Tax charged/(credited) to other comprehensive income and equity 2018 2017 2016 £m £m £m Current tax: Share-based payments (3 ) (4 ) (1 ) Available-for-sale investments (11 ) 6 5 Deferred tax: Available-for-sale investments (18 ) 8 12 Cash flow hedges (4 ) 20 22 Share-based payments 1 1 — Remeasurements of gains of pension assets and post-retirement benefit obligations 1 530 277 95 495 308 133 Total tax recognised in the statements of comprehensive income from continuing operations 497 311 134 Total tax recognised in the statements of comprehensive income from discontinued operations — 10 23 Total tax relating to share-based payments recognised directly in equity from continuing operations (2 ) (3 ) (1 ) Total tax relating to share-based payments recognised directly in equity from discontinued operations — — (1 ) 495 318 155 1. Remeasurements of gains of pension assets and post-retirement benefit obligations includes a deferred tax charge of £281 million arising on the reduction in the US corporation tax rate.
Disclosure Of Reconciliation Of Accounting Profit Multiplied By Applicable Tax Rates And Average Effective Tax Rate	The tax credit for the year after exceptional items and remeasurements, for the continuing business, is lower ( 2017 : lower tax charge; 2016 : lower tax charge) than the standard rate of corporation tax in the UK of 19% ( 2017 : 20% ; 2016 : 20% ): Before exceptional items and remeasurements After exceptional items and remeasurements Before exceptional items and remeasurements After exceptional items and remeasurements Before exceptional items and remeasurements After exceptional items and remeasurements 2018 2018 2017 2017 2016 2016 £m £m £m £m £m £m Profit before tax from continuing operations Before exceptional items and remeasurements 2,650 2,650 2,807 2,807 2,417 2,417 Exceptional items and remeasurements — 58 — (623 ) — (88 ) Profit before tax from continuing operations 2,650 2,708 2,807 2,184 2,417 2,329 Profit before tax from continuing operations multiplied by UK corporation tax rate of 19% (2017: 20%; 2016: 20%) 503 515 561 437 483 465 Effect of: Adjustments in respect of prior years 1 (22 ) (14 ) (67 ) (67 ) 2 1 Expenses not deductible for tax purposes 2 20 21 35 442 25 114 Non-taxable income 2 (16 ) (47 ) (24 ) (425 ) (25 ) (112 ) Adjustment in respect of foreign tax rates 153 157 180 104 124 129 Deferred tax impact of change in UK tax rate (7 ) (7 ) — (94 ) — (162 ) Deferred tax impact of change in US tax rate due to Tax Reform — (1,510 ) — — — — Other 3 (42 ) 1 (19 ) (23 ) (5 ) (8 ) Total tax charge/(credit) from continuing operations 589 (884 ) 666 374 604 427 % % % % % % Effective tax rate – continuing operations 22.2 (32.6 ) 23.7 17.1 25.0 18.3 1. Prior year adjustment is primarily due to agreement of prior period tax returns. 2. For the years ended 31 March 2017 and prior, the adjustments after exceptional items and remeasurements primarily represent the impact of the Group’s net investment hedging following significant US dollar currency fluctuations. 3. Other primarily comprises tax on joint ventures and associates.
Disclosure of temporary difference, unused tax losses and unused tax credits	The deferred tax assets not recognised are as follows: 2018 2017 £m £m Capital losses 510 362 Non-trade deficits 4 4 Trading losses 4 9 The following are the major deferred tax assets and liabilities recognised, and the movements thereon, during the current and prior reporting periods: Accelerated Share- Pensions Financial Other net 1 Total £m £m £m £m £m £m Deferred tax liabilities/(assets) At 1 April 2016 7,063 (14 ) (1,038 ) (53 ) (1,324 ) 4,634 Exchange adjustments and other 2 681 1 (144 ) (7 ) (50 ) 481 Charged/(credited) to income statement 402 — 177 23 (481 ) 121 Charged to other comprehensive income and equity — 1 264 46 5 316 Disposal of UK Gas Distribution (1,072 ) — (6 ) — 5 (1,073 ) At 1 April 2017 7,074 (12 ) (747 ) 9 (1,845 ) 4,479 Exchange adjustments and other 2 (559 ) — 69 1 221 (268 ) (Credited)/charged to income statement (1,641 ) 2 (55 ) 12 598 (1,084 ) Charged/(credited) to other comprehensive income and equity — 1 530 (1 ) (21 ) 509 At 31 March 2018 4,874 (9 ) (203 ) 21 (1,047 ) 3,636 1. The deferred tax asset of £1,047 million as at 31 March 2018 in respect of other net temporary differences primarily relates to US net operating losses ( £390 million ) and environmental provisions ( £378 million ). 2. Exchange adjustments and other comprises of foreign exchange arising on translation of the US dollar deferred tax balances together with a reclassification of £43 million (2017: £143 million ) being the opening deferred tax balance in respect of US net operating losses to offset against US current tax liabilities.
Disclosure Of Income Taxes Borne and Collected	UK total tax contribution 2017 /18 (taxes paid/collected) Taxes borne £m 1. VAT 2 2. PAYE and NIC 52 3. UK corporation tax 37 4. Business rates 222 5. Other 23 Total 336 Taxes collected £m 1. VAT 478 2. PAYE and NIC 125 Total 603
Disclosure Of Reconciliation Of Income Tax Charges To Income Taxes Paid	The tax credit for the Group from continuing operations as reported in the income statement is £884 million ( 2016 /17: £374 million charge). The UK tax charge is £250 million ( 2016 /17: £151 million ) and UK corporation tax paid was £37 million ( 2016 /17: £129 million ), with the principal differences between these two measures as follows: Year ended 31 March 2018 2017 Reconciliation on continuing operations of UK total tax charge to UK corporation tax paid £m £m Total UK tax charge (current tax £187m 250 151 Adjustment for non-cash deferred tax (63 ) 27 Adjustments for current tax credit in respect of prior years 18 47 UK current tax charge 205 225 UK corporation tax instalment payments not payable until the following year (101 ) (216 ) UK corporation tax instalment (refunds)/payments in respect of prior years paid in current year (67 ) 120 UK corporation tax paid 37 129

Earnings per share (EPS) (Table

Earnings per share (EPS) (Tables)	12 Months Ended
	Mar. 31, 2018
Earnings per share [abstract]
Schedule of earnings per share	(a) Basic earnings per share Earnings Earnings per share Earnings Earnings per share Earnings Earnings per share 2018 2018 2017 2017 2016 2016 £m pence £m pence £m pence Adjusted earnings from continuing operations 2,060 59.5 2,141 56.9 1,812 48.0 Exceptional items after tax from continuing operations 1,319 38.1 (312 ) (8.3 ) 162 4.3 Remeasurements after tax from continuing operations 212 6.2 (19 ) (0.5 ) (73 ) (1.9 ) Earnings from continuing operations 3,591 103.8 1,810 48.1 1,901 50.4 Adjusted earnings from discontinued operations — — 607 16.1 574 15.2 Exceptional items and remeasurements after tax from discontinued operations (41 ) (1.2 ) 5,378 142.9 116 3.1 Earnings from discontinued operations (41 ) (1.2 ) 5,985 159.0 690 18.3 Total adjusted earnings 2,060 59.5 2,748 73.0 2,386 63.2 Total exceptional items and remeasurements after tax (including discontinued operations) 1,490 43.1 5,047 134.1 205 5.5 Total earnings 3,550 102.6 7,795 207.1 2,591 68.7 2018 2017 2016 millions millions millions Weighted average number of ordinary shares – basic 3,461 3,763 3,774 (b) Diluted earnings per share Earnings Earnings per share Earnings Earnings per share Earnings Earnings per share 2018 2018 2017 2017 2016 2016 £m pence £m pence £m pence Adjusted earnings from continuing operations 2,060 59.3 2,141 56.7 1,812 47.8 Exceptional items after tax from continuing operations 1,319 37.9 (312 ) (8.3 ) 162 4.3 Remeasurements after tax from continuing operations 212 6.1 (19 ) (0.5 ) (73 ) (1.9 ) Earnings from continuing operations 3,591 103.3 1,810 47.9 1,901 50.2 Adjusted earnings from discontinued operations — — 607 16.0 574 15.1 Exceptional items and remeasurements after tax from discontinued operations (41 ) (1.2 ) 5,378 142.3 116 3.1 Earnings from discontinued operations (41 ) (1.2 ) 5,985 158.3 690 18.2 Total adjusted earnings 2,060 59.3 2,748 72.7 2,386 62.9 Total exceptional items and remeasurements after tax (including discontinued operations) 1,490 42.8 5,047 133.5 205 5.5 Total earnings 3,550 102.1 7,795 206.2 2,591 68.4 2018 2017 2016 millions millions millions Weighted average number of ordinary shares – diluted 3,476 3,780 3,790 (c) Reconciliation of basic to diluted average number of shares 2018 2017 2016 millions millions millions Weighted average number of ordinary shares – basic 3,461 3,763 3,774 Effect of dilutive potential ordinary shares – employee share plans 15 17 16 Weighted average number of ordinary shares – diluted 3,476 3,780 3,790

Dividends (Tables)

Dividends (Tables)	12 Months Ended
	Mar. 31, 2018
Subclassifications of assets, liabilities and equities [abstract]
Schedule of Dividends to Shareholders	Final dividends are recognised when they are approved by shareholders. 2018 2017 2016 Pence per share Cash dividend paid £m Scrip dividend £m Pence per share Cash dividend paid £m Scrip dividend £m Pence per share Cash dividend paid £m Scrip dividend £m Interim dividend in respect of the current year 15.49 346 176 15.17 540 32 15.00 532 31 Special dividend 84.375 3,171 — — — — — — — Final dividend in respect of the prior year 29.10 970 33 28.34 923 151 28.16 805 248 128.965 4,487 209 43.51 1,463 183 43.16 1,337 279

Discontinued operations (Tables

Discontinued operations (Tables)	12 Months Ended
	Mar. 31, 2018
Non-current Assets Held For Sale And Discontinued Operations [Abstract]
Schedule of discontinued operations	2017 2016 Notes £m £m Profit after tax from discontinued operations 5,984 692 Other comprehensive (loss)/income Items that will never be reclassified to profit or loss: Remeasurement (losses)/gains of pension assets and post-retirement benefit obligations 23 (75 ) 129 Tax on items that will never be reclassified to profit or loss 6 13 (30 ) Total items from discontinued operations that will never be reclassified to profit or loss (62 ) 99 Items that may be reclassified subsequently to profit or loss: Net losses in respect of cash flow hedges (106 ) (38 ) Transferred to profit or loss in respect of cash flow hedges 233 3 Tax on items that may be reclassified subsequently to profit or loss 6 (23 ) 7 Total items from discontinued operations that may be reclassified subsequently to profit or loss 104 (28 ) Other comprehensive income/(loss) for the year, net of tax from discontinued operations 42 71 Total comprehensive income for the year from discontinued operations 6,026 763 The summary income statement for discontinued operations for the years ended 31 March 2017 and 2016 are as follows: 2017 2016 £m £m Revenue 1,887 1,903 Operating costs (993 ) (1,043 ) Operating profit 1 894 860 Finance costs (152 ) (157 ) Profit before tax from discontinued operations 742 703 Tax from discontinued operations (79 ) (11 ) Profit after tax from discontinued operations 663 692 Gain on disposal of UK Gas Distribution 5,009 — Tax on gain on disposal of UK Gas Distribution 312 — Gain on disposal of UK Gas Distribution after tax 5,321 — Total profit after tax from discontinued operations 5,984 692 1. 2016 includes sale preparation costs of £22 million in respect of the disposal of the UK Gas Distribution business. 2017 costs have been included as part of transaction costs in determining the gain on disposal.

Goodwill (Tables)

Goodwill (Tables)	12 Months Ended
	Mar. 31, 2018
Intangible Assets [Abstract]
Schedule of changes in intangible assets and goodwill	Impairments are recognised in the income statement and are disclosed separately. Total £m Net book value at 1 April 2016 5,315 Exchange adjustments 781 Net book value at 31 March 2017 6,096 Exchange adjustments (652 ) Net book value at 31 March 2018 5,444

Other intangible assets (Tables

Other intangible assets (Tables)	12 Months Ended
	Mar. 31, 2018
Intangible Assets [Abstract]
Schedule of intangible assets	Other intangible assets are amortised on a straight-line basis over their estimated useful economic lives. Amortisation periods for categories of intangible assets are: Years Software 3 to 10 Software £m Cost at 1 April 2016 1,744 Exchange adjustments 105 Additions 234 Disposals (43 ) Disposal of UK Gas Distribution (304 ) Reclassifications 1 (4 ) Cost at 31 March 2017 1,732 Exchange adjustments (98 ) Additions 173 Disposals (18 ) Reclassifications 1 8 Cost at 31 March 2018 1,797 Accumulated amortisation at 1 April 2016 (857 ) Exchange adjustments (43 ) Amortisation charge for the year (164 ) Accumulated amortisation of disposals 40 Disposal of UK Gas Distribution 215 Accumulated amortisation at 31 March 2017 (809 ) Exchange adjustments 43 Amortisation charge for the year (138 ) Accumulated amortisation of disposals 6 Accumulated amortisation at 31 March 2018 (898 ) Net book value at 31 March 2018² 899 Net book value at 31 March 2017 923 1. Reclassifications includes amounts transferred (to)/from property, plant and equipment (see note 12). 2. Included in software is £160 million relating to the US ERP system, which still has a remaining amortisation period of six years.

Property, plant and equipment (

Property, plant and equipment (Tables)	12 Months Ended
	Mar. 31, 2018
Property, plant and equipment [abstract]
Schedule of property, plant and equipment	Land and buildings £m Plant and machinery £m Assets in the course of construction £m Motor vehicles and office equipment £m Total £m Cost at 1 April 2016 2,758 54,772 3,874 1,171 62,575 Exchange adjustments 196 3,157 93 76 3,522 Additions 55 822 3,080 132 4,089 Disposals 1 (22 ) (572 ) (70 ) (204 ) (868 ) Disposal of UK Gas Distribution (112 ) (11,861 ) (88 ) (300 ) (12,361 ) Reclassifications 2 104 2,913 (2,938 ) (41 ) 38 Cost at 31 March 2017 2,979 49,231 3,951 834 56,995 Exchange adjustments (169 ) (2,862 ) (89 ) (67 ) (3,187 ) Additions 38 430 3,358 75 3,901 Disposals 1 (16 ) (216 ) (21 ) (34 ) (287 ) Reclassifications 2 98 2,791 (2,926 ) 49 12 Cost at 31 March 2018 2,930 49,374 4,273 857 57,434 Accumulated depreciation at 1 April 2016 (640 ) (17,828 ) — (743 ) (19,211 ) Exchange adjustments (29 ) (780 ) — (44 ) (853 ) Depreciation charge for the year (84 ) (1,338 ) — (113 ) (1,535 ) Disposals 1 42 545 — 203 790 Disposal of UK Gas Distribution 29 3,425 — 207 3,661 Reclassifications 2 (2 ) (20 ) — — (22 ) Accumulated depreciation at 31 March 2017 (684 ) (15,996 ) — (490 ) (17,170 ) Exchange adjustments 28 695 — 36 759 Depreciation charge for the year (28 ) (1,276 ) — (88 ) (1,392 ) Disposals 1 10 199 — 33 242 Reclassifications 2 — (20 ) — — (20 ) Accumulated depreciation at 31 March 2018 (674 ) (16,398 ) — (509 ) (17,581 ) Net book value at 31 March 2018 2,256 32,976 4,273 348 39,853 Net book value at 31 March 2017 2,295 33,235 3,951 344 39,825 1. Includes the reversal of assets with cost of £51 million ( 2017 : £107 million ) and accumulated depreciation of £51 million ( 2017 : £107 million ) disposed in previous years that remain in use in the Group. It also includes £334 million of adjustments from accumulated depreciation to cost for historical disposals relating to assets acquired as part of the KeySpan acquisition in 2008 which were disposed of in subsequent periods. Both of these adjustments have a nil net book value impact. 2. Represents amounts transferred between categories, (to)/from other intangible assets (see note 11), reclassifications from inventories and reclassifications between cost and accumulated depreciation. 2018 2017 £m £m Information in relation to property, plant and equipment Capitalised interest included within cost 1,861 1,749 Net book value of assets held under finance leases (all relating to motor vehicles and office equipment) 253 289 Additions to assets held under finance leases (all relating to motor vehicles and office equipment) 58 98 Contributions to cost of property, plant and equipment included within: Trade and other payables 85 89 Non-current liabilities 844 839 Unless otherwise determined by operational requirements, the depreciation periods for the principal categories of property, plant and equipment are, in general, as shown in the table below: Years Freehold and leasehold buildings up to 101 Plant and machinery: Electricity transmission plant and wires 15 to 10 0 Electricity distribution plant 29 to 75 Electricity generation plant 20 to 93 Interconnector plant and other 5 to 60 Gas plant – mains, services and regulating equipment 10 to 95 Gas plant – storage 5 to 65 Gas plant – meters 7 to 65 Motor vehicles and office equipment up to 29

Other non-current assets (Table

Other non-current assets (Tables)	12 Months Ended
	Mar. 31, 2018
Subclassifications of assets, liabilities and equities [abstract]
Schedule of other non current assets	2018 2017¹ £m £m Other receivables 36 45 Non-current tax assets 51 — Prepayments and accrued income 28 24 115 69 1. Comparative amounts have been represented to reflect the reclassification of commodity derivative contracts from other non-current assets to derivatives (see note 16).

Financial and other investmen68

Financial and other investments (Tables)	12 Months Ended
	Mar. 31, 2018
Disclosure of financial assets [abstract]
Schedule of financial assets	2018 2017 £m £m Non-current Available-for-sale investments 417 605 Loans to joint ventures and associates 1 482 495 899 1,100 Current Available-for-sale investments 2,304 7,432 Other loans and receivables 390 1,309 2,694 8,741 3,593 9,841 Financial and other investments include the following: Investments in short-term money funds 2 1,999 6,899 Managed investments in equity and bonds 3 530 939 Cash surrender value of life insurance policies 198 202 Loans to joint ventures and associates 482 495 Restricted balances: Collateral 4 335 1,262 Other 49 44 3,593 9,841 1. Comprises £352 million ( 2017 : £434 million ) relating to a shareholder loan to Quadgas HoldCo Limited, and the remainder is a loan to a joint venture. 2. Includes £69 million ( 2017 : £14 million ) held by insurance captives and therefore restricted. 3. Includes restricted amounts of £301 million ( 2017 : £434 million ) held by insurance captives and £214 million ( 2017 : £225 million ) relating to US non-qualified plan investments. 4. Refers to collateral placed with counterparties with whom we have entered into a credit support annex to the ISDA (International Swaps and Derivatives Association) Master Agreement.

Investments in joint ventures69

Investments in joint ventures and associates (Tables)	12 Months Ended
	Mar. 31, 2018
Interests In Other Entities [Abstract]
Disclosure of joint ventures	Summarised financial information of this joint venture, as at 31 March, together with the carrying amount of the investment in the consolidated financial statements is as follows: 2018 2017 £m £m Statement of financial position – BritNed Development Limited Non-current assets 390 392 Cash and cash equivalents 50 45 All other current assets 4 1 Non-current liabilities (10 ) (10 ) Current liabilities (28 ) (20 ) Equity 406 408 Carrying amount of the Group’s investment (National Grid ownership 50%) 203 204 2018 2017 £m £m Income statement – BritNed Development Limited Revenue 429 399 Depreciation and amortisation (13 ) (13 ) Other costs (324 ) (257 ) Operating profit 92 129 Income tax expense (20 ) (23 ) Profit for the year 72 106 Group’s share of profit (National Grid ownership 50%) 36 53 Investments in joint ventures and associates represent businesses we do not control, but instead exercise joint control or significant influence. A joint venture is an arrangement established to engage in economic activity, which the Group jointly controls with other parties and has rights to the net assets of the arrangement. An associate is an entity which is neither a subsidiary nor a joint venture, but over which the Group has significant influence. 2018 2017 Associates £m Joint ventures £m Total £m Associates £m Joint ventures £m Total £m Share of net assets at 1 April 1,776 307 2,083 84 313 397 Exchange adjustments (19 ) 7 (12 ) 16 19 35 Additions 65 64 129 74 63 137 Acquisition of stake in Quadgas HoldCo Limited — — — 1,611 — 1,611 Capitalisation of shareholder loan to Quadgas HoldCo Limited 69 — 69 — — — Impairment charge against investment in Quadgas HoldCo Limited (213 ) — (213 ) — — — Share of post-tax results for the year 147 26 173 15 48 63 Share of other comprehensive income of associates, net of tax 147 — 147 — — — Dividends received (170 ) (43 ) (213 ) (24 ) (75 ) (99 ) Other movements 5 — 5 — (61 ) (61 ) Share of net assets at 31 March 1,807 361 2,168 1,776 307 2,083
Disclosure of associates	The fair values of the assets and liabilities of Quadgas HoldCo Limited as at 31 March 2017 , as previously reported, are set out below, along with the reconciliation to the carrying value of the investment in the associate at that date: 2017 £m Statement of financial position – Quadgas HoldCo Limited (as previously reported) Non-current assets 15,559 Current assets 299 Non-current liabilities (10,408 ) Current liabilities (519 ) Equity 4,931 Proportion of the Group’s ownership interest in associate 1,923 Discount for non-controlling interest (312 ) Carrying amount of the Group’s interest in associate (National Grid ownership 39%) 1,611 Summarised financial information of this associate (incorporating the purchase price allocation completion adjustments), as at 31 March, together with the carrying amount of the investment in the consolidated financial statements is as follows: 2018 2017 £m £m Statement of financial position – Quadgas HoldCo Limited Non-current assets 16,735 16,047 Current assets 427 299 Non-current liabilities (11,195 ) (10,864 ) Current liabilities (534 ) (551 ) Equity 5,433 4,931 Proportion of the Group’s ownership interest in associate 2,119 1,923 Discount for non-controlling interest (arising on initial acquisition) (312 ) (312 ) Impairment charge against investment (213 ) — Carrying amount of the Group’s interest in associate (National Grid ownership 39%) 1,594 1,611 The sale of the previously owned subsidiary and subsequent acquisition of the 39% equity interest occurred (see note 9) on 31 March 2017 . All profit or loss impact for the year ended 31 March 2017 is disclosed as discontinued operations. The summarised income statement for the year ended 31 March 2018 is set out below: 2018 £m Income statement – Quadgas HoldCo Limited Revenue 1,468 Depreciation and amortisation (378 ) Other costs (423 ) Operating profit 667 Net interest payable (272 ) Income tax expense (76 ) Profit for the year 319 Group’s share of profit (National Grid ownership 39%) 124 A joint venture is an arrangement established to engage in economic activity, which the Group jointly controls with other parties and has rights to the net assets of the arrangement. An associate is an entity which is neither a subsidiary nor a joint venture, but over which the Group has significant influence. 2018 2017 Associates £m Joint ventures £m Total £m Associates £m Joint ventures £m Total £m Share of net assets at 1 April 1,776 307 2,083 84 313 397 Exchange adjustments (19 ) 7 (12 ) 16 19 35 Additions 65 64 129 74 63 137 Acquisition of stake in Quadgas HoldCo Limited — — — 1,611 — 1,611 Capitalisation of shareholder loan to Quadgas HoldCo Limited 69 — 69 — — — Impairment charge against investment in Quadgas HoldCo Limited (213 ) — (213 ) — — — Share of post-tax results for the year 147 26 173 15 48 63 Share of other comprehensive income of associates, net of tax 147 — 147 — — — Dividends received (170 ) (43 ) (213 ) (24 ) (75 ) (99 ) Other movements 5 — 5 — (61 ) (61 ) Share of net assets at 31 March 1,807 361 2,168 1,776 307 2,083

Derivative financial instrume70

Derivative financial instruments (Tables)	12 Months Ended
	Mar. 31, 2018
Financial Instruments [Abstract]
Disclosure of detailed information about derivative financial instruments	For each class of commodity contract derivative type, the fair value amounts are as follows: 2018 2017 Assets £m Liabilities £m Total £m Assets Liabilities £m Total £m Commodity purchase contracts accounted for as derivative contracts Forward purchases of electricity — — — — (10 ) (10 ) Forward purchases of gas 60 (64 ) (4 ) 82 (97 ) (15 ) Derivative financial instruments linked to commodity prices Electricity capacity 1 — 1 2 — 2 Electricity swaps 7 (46 ) (39 ) 11 (61 ) (50 ) Electricity options — (1 ) (1 ) — — — Gas swaps 1 (4 ) (3 ) 11 (2 ) 9 Gas options — (1 ) (1 ) — — — 69 (116 ) (47 ) 106 (170 ) (64 ) The total fair value of derivatives split by category is as follows: 2018 2017 Assets £m Liabilities £m Total £m Assets £m Liabilities £m Total £m Financing derivatives 1,545 (945 ) 600 1,707 (2,223 ) (516 ) Commodity derivatives 1 69 (116 ) (47 ) 106 (170 ) (64 ) Further Acquisition Agreement derivative 2 110 — 110 — — — 1,724 (1,061 ) 663 1,813 (2,393 ) (580 ) 1. Comparative amounts have been re-presented to reflect the reclassification of commodity derivative contracts from trade and other receivables ( 31 March 2017 : £54 million ), trade and other payables ( 31 March 2017 : £93 million ), other non-current assets ( 31 March 2017 : £52 million ) and other non-current liabilities ( 31 March 2017 : £77 million ), to current and non-current derivative financial assets and derivative financial liabilities. 2. This year a further derivative category has been added for the Further Acquisition Agreement (FAA) derivative. This relates to the put/call option over a 14% interest in Quadgas HoldCo Limited. Refer to note 15 for further details. For each class of financing derivative instrument type the fair value amounts are as follows: 2018 2017 Assets £m Liabilities £m Total £m Assets £m Liabilities £m Total £m Interest rate swaps 678 (457 ) 221 849 (657 ) 192 Cross-currency interest rate swaps 687 (207 ) 480 676 (909 ) (233 ) Foreign exchange forward contracts 1 174 (2 ) 172 160 (113 ) 47 Inflation linked swaps 5 (278 ) (273 ) 7 (529 ) (522 ) Equity options 1 (1 ) — 15 (15 ) — 1,545 (945 ) 600 1,707 (2,223 ) (516 ) 1. Included within the foreign exchange forward contracts balance is £67 million ( 2017 : £69 million ) of derivatives in relation to hedging of capital expenditure. For each class of commodity contract derivative, our exposure based on the notional quantities is as follows: 2018 2017 Forward purchases of electricity 1 0 GWh 159 GWh Forward purchases of gas 2 54m Dth 54m Dth Electricity swaps 12,839 GWh 12,776 GWh Electricity options 13,897 GWh 17,793 GWh Electricity capacity 0.6 GWm 0.7 GWm Gas swaps 100m Dth 83m Dth Gas options 7m Dth 9m Dth NYMEX gas futures 3 0 Dth 3m Dth 1. Forward electricity purchases expired on 30 September 2017. The contractual obligations under these contracts are £nil ( 2017 : £15 million ). 2. Forward gas purchases have terms up to three years . The contractual obligations under these contracts are £96 million ( 2017 : £131 million ). 3. NYMEX gas futures have been offset with related margin accounts (see note 30(a)). For each class of financing derivative the notional contract 1 amounts are as follows: 2018 2017 £m £m Interest rate swaps (8,390 ) (9,469 ) Cross-currency interest rate swaps (6,925 ) (8,631 ) Foreign exchange forward contracts (5,793 ) (8,253 ) Inflation linked swaps (1,191 ) (1,423 ) Equity options (800 ) (800 ) (23,099 ) (28,576 ) 1. The notional contract amounts of derivatives indicate the gross nominal value of transactions outstanding at the reporting date.
Disclosure of maturity analysis for derivative financial liabilities	The maturity profile of commodity contracts is as follows: 2018 2017 Assets £m Liabilities £m Total £m Assets £m Liabilities £m Total £m Current Less than one year 30 (76 ) (46 ) 54 (93) (39) 30 (76 ) (46 ) 54 (93) (39) Non-current In 1 to 2 years 6 (17 ) (11 ) 8 (36) (28) In 2 to 3 years 6 (11 ) (5 ) 7 (9) (2) In 3 to 4 years 6 (3 ) 3 6 (7) (1) In 4 to 5 years 5 (2 ) 3 6 (5) 1 More than 5 years 16 (7 ) 9 25 (20) 5 39 (40 ) (1 ) 52 (77) (25) 69 (116 ) (47 ) 106 (170) (64) The maturity profile of financing derivative instruments is as follows: 2018 2017 Assets £m Liabilities £m Total £m Assets £m Liabilities £m Total Current Less than 1 year 375 (325 ) 50 192 (1,054 ) (862 ) 375 (325 ) 50 192 (1,054 ) (862 ) Non-current In 1 to 2 years 83 (88 ) (5 ) 199 (305 ) (106 ) In 2 to 3 years 25 (27 ) (2 ) 122 (160 ) (38 ) In 3 to 4 years 418 (5 ) 413 39 (83 ) (44 ) In 4 to 5 years 12 — 12 419 (36 ) 383 More than 5 years 632 (500 ) 132 736 (585 ) 151 1,170 (620 ) 550 1,515 (1,169 ) 346 1,545 (945 ) 600 1,707 (2,223 ) (516 )
Disclosure of detailed information about hedging instruments	Fair value hedges Fair value hedges principally consist of interest rate and cross-currency swaps that are used to protect against changes in the fair value of fixed-rate, long-term financial instruments due to movements in market interest rates and foreign exchange rates. For qualifying fair value hedges, all changes in the fair value of the derivative and changes in the fair value of the item in relation to the risk being hedged are offset in the income statement to the extent the fair value hedge is effective. Adjustments made to the carrying amount of the hedged item for fair value hedges will be amortised over the remaining life, in line with the hedged item. 2018 2017 £m £m Cross-currency interest rate/interest rate swaps 496 548 16. Derivative financial instruments continued (d) Hedge accounting continued Cash flow hedges Exposure arises from the variability in future interest and currency cash flows on assets and liabilities which bear interest at variable rates or are in a foreign currency. Interest rate and cross-currency swaps are maintained, and designated as cash flow hedges, where they qualify, to manage this exposure. Fair value changes on designated cash flow hedges are initially recognised in other comprehensive income as gains or losses recognised in equity and transferred to the cash flow hedge reserve, and any ineffective portion is recognised immediately in the income statement. Amounts are transferred from equity and recognised in the income statement as the income or expense is recognised on the hedged item. Forward foreign currency contracts are used to hedge anticipated and committed future currency cash flows. Where these contracts qualify for hedge accounting they are designated as cash flow hedges. On recognition of the underlying transaction in the financial statements, the associated hedge gains and losses, deferred in equity, are transferred and included with the recognition of the underlying transaction. When a forecast transaction is no longer expected to occur, the cumulative gain or loss previously reported in equity is transferred to the income statement. Where a non-financial asset or a non-financial liability results from a forecast transaction or firm commitment being hedged, the amounts deferred in equity are included in the initial measurement of that non-monetary asset or liability. 2018 2017 £m £m Cross-currency interest rate/interest rate swaps 138 (180 ) Foreign exchange forward contracts 66 69 204 (111 ) Net investment hedges Borrowings, cross-currency swaps and forward currency contracts are used in the management of the foreign exchange exposure arising from the investment in non-sterling denominated subsidiaries. Where these contracts qualify for hedge accounting they are designated as net investment hedges. 2018 2017 £m £m Cross-currency interest rate/interest rate swaps (104 ) (544 ) Foreign exchange forward contracts 87 (56 ) (17 ) (600 )
Disclosure of detailed information about derivatives not in a formal hedge relationship	Derivatives not in a formal hedge relationship Our policy is not to use derivatives for trading purposes. However, due to the complex nature of hedge accounting under IAS 39 some derivatives may not qualify for hedge accounting, or are specifically not designated as a hedge where natural offset is more appropriate. Changes in the fair value of any derivative instruments that do not qualify for hedge accounting are recognised in remeasurements within the income statement. 2018 2017 £m £m Cross-currency interest rate/interest rate swaps 171 135 Foreign exchange forward contracts 19 34 Inflation linked swaps (273 ) (522 ) Equity options — — Commodity derivatives (all categories) (47 ) (64 ) Further Acquisition Agreement derivative 110 — (20 ) (417 )

Inventories and other current71

Inventories and other current intangibles (Tables)	12 Months Ended
	Mar. 31, 2018
Subclassifications of assets, liabilities and equities [abstract]
Schedule of inventories	A liability is recorded in respect of the obligation to deliver emission allowances, and emission charges are recognised in the income statement in the period in which emissions are made. 2018 2017 £m £m Fuel stocks 78 101 Raw materials and consumables 190 191 Work in progress — 8 Current intangible assets – emission allowances 73 103 341 403

Trade and other receivables (Ta

Trade and other receivables (Tables)	12 Months Ended
	Mar. 31, 2018
Trade and other receivables [abstract]
Schedule of trade and other receivables	A provision is established for irrecoverable amounts when there is objective evidence that amounts due under the original payment terms will not be collected. 2018 2017¹ £m £m Trade receivables 1,674 1,591 Accrued income 817 811 Prepayments 229 228 Other receivables 78 98 2,798 2,728 1. Comparative amounts have been represented to reflect the reclassification of commodity contract derivatives from trade and other receivables to derivatives (see note 16).
Schedule of reconciliation of changes in allowance account for credit losses	Provision for impairment of receivables 2018 2017 £m £m At 1 April 424 349 Exchange adjustments (42 ) 51 Charge for the year, net of recoveries 36 147 Uncollectible amounts written off against receivables (109 ) (121 ) Disposal of UK Gas Distribution — (2 ) At 31 March 309 424
Schedule of financial assets that are either past due or impaired	Trade receivables past due but not impaired 2018 2017 £m £m Up to 3 months past due 271 238 3 to 6 months past due 73 67 Over 6 months past due 131 143 475 448

Cash and cash equivalents (Tabl

Cash and cash equivalents (Tables)	12 Months Ended
	Mar. 31, 2018
Cash and cash equivalents [abstract]
Schedule of cash and cash equivalents	For further information on currency exposures, refer to note 30(d). 2018 2017 £m £m Cash at bank 54 199 Short-term deposits 275 940 Cash and cash equivalents 329 1,139

Borrowings (Tables)

Borrowings (Tables)	12 Months Ended
	Mar. 31, 2018
Disclosure of detailed information about borrowings [abstract]
Schedule of borrowings	2018 2017 £m £m Current Bank loans 2,020 1,339 Bonds 2,156 2,209 Commercial paper 206 1,881 Finance leases 64 66 Other loans 1 1 4,447 5,496 Non-current Bank loans 2,384 2,343 Bonds 19,418 20,368 Finance leases 207 242 Other loans 169 189 22,178 23,142 Total borrowings 26,625 28,638 20. Borrowings continued Total borrowings are repayable as follows: 2018 2017 £m £m Less than 1 year 4,447 5,496 In 1 to 2 years 1,694 1,941 In 2 to 3 years 2,347 1,821 In 3 to 4 years 1,873 2,453 In 4 to 5 years 1,469 1,921 More than 5 years: By instalments 1,010 1,043 Other than by instalments 13,785 13,963 26,625 28,638 Maturity analysis of parent Company borrowings 2018 2017 £m £m Total borrowings are repayable as follows: Less than 1 year 781 1,120 In 1 to 2 years 438 515 In 2 to 3 years — 425 In 3 to 4 years 335 — In 4 to 5 years — 322 More than 5 years — — 1,554 2,382
Schedule of finance lease obligations	Assets held under finance leases are depreciated over the shorter of their useful life and the lease term. Finance lease obligations 2018 2017 £m £m Gross finance lease liabilities are repayable as follows: Less than 1 year 64 66 1 to 5 years 177 213 More than 5 years 72 89 313 368 Less: finance charges allocated to future periods (42 ) (60 ) 271 308 The present value of finance lease liabilities is as follows: Less than 1 year 64 66 1 to 5 years 144 174 More than 5 years 63 68 271 308

Trade and other payables (Table

Trade and other payables (Tables)	12 Months Ended
	Mar. 31, 2018
Subclassifications of assets, liabilities and equities [abstract]
Schedule of trade and other payables	Trade and other payables include amounts owed to suppliers, tax authorities and other parties which are due to be settled within 12 months. The total also includes deferred income, which represents monies received from customers but for which we have not yet delivered the associated service. These amounts are recognised as revenue when the service is provided. Trade and other payables are initially recognised at fair value and subsequently measured at amortised cost. 2018 2017¹ £m £m Trade payables 1,977 2,135 Deferred income 2 440 298 Social security and other taxes 173 136 Other payables 863 776 3,453 3,345 1. Comparative amounts have been represented to reflect the reclassification of commodity derivative contracts from trade and other payables to derivatives (see note 16). 2. Included within deferred income is £85 million ( 2017 : £89 million ) relating to customer contributions.

Other non-current liabilities (

Other non-current liabilities (Tables)	12 Months Ended
	Mar. 31, 2018
Subclassifications of assets, liabilities and equities [abstract]
Schedule of other non current liabilities	Non-current liabilities are initially recognised at fair value and subsequently measured at amortised cost. 2018 2017¹ £m £m Deferred income 2 958 1,032 Other payables 359 338 1,317 1,370 1. Comparative amounts have been represented to reflect the reclassification of commodity derivative contracts from other non-current liabilities to derivatives (see note 16). 2. Included within deferred income is £844 million ( 2017 : £839 million ) relating to customer contributions. There is no material difference between the fair value and the carrying value of other payables.

Pensions and other post retir77

Pensions and other post retirement benefits (Tables)	12 Months Ended
	Mar. 31, 2018
Employee Benefits [Abstract]
Schedule of results of most recent actuarial valuations	The results of the most recent actuarial valuations are shown below. See page 145 for the assumptions used for IAS 19 purposes. Section A of NGUKPS Section B of NGUKPS NGEG of ESPS Latest full actuarial valuation 31 March 2017 31 March 2017 31 March 2016 Actuary Willis Towers Watson Willis Towers Watson Aon Hewitt Market value of scheme assets at latest valuation £6,716 m £5,849 m £2,553 m Actuarial value of benefits due to members £6,627 m £6,057 m £3,053 m Market value as percentage of benefits 101% 97% 84% Funding surplus/(deficit) £89 m (£208m) (£500m) Funding surplus/(deficit) net of tax £74 m (£173m) (£415m)
Disclosure of actuarial assumptions	The Company has applied the following financial assumptions in assessing DB liabilities. UK pensions 2018 2017 2016 % % % Discount rate 1 – past service 2 2.60 2.40 3.30 Discount rate 1 – future service 2 2.65 2.65 3.30 Rate of increase in salaries 3 3.40 3.50 3.20 Rate of increase in RPI – past service 4 3.15 3.20 2.90 Rate of increase in RPI – future service 4 3.10 3.15 2.90 1. The discount rates for pension liabilities have been determined by reference to appropriate yields on high-quality corporate bonds prevailing in the UK debt markets at the reporting date. 2. In the UK for 2018, National Grid has adopted a different discount rate assumption by increasing the duration of the scheme liabilities to 25 years for future service obligations. This has led to a future service discount rate in the UK of 2.65% for both the 2017 and 2018 year-ends. The 2017 discount rate was 2.40% based on an expected duration of scheme liabilities of 17 years . 3. A promotional scale has also been used where appropriate. The UK assumption stated is that relating to service prior to 1 April 2013. The UK assumption for the rate of increase in salaries for past service after this date is 2.20% (2017: 2.20% ; 2016: 2.10% ) and for future service 2.15% (2017: 2.20% ; 2016: 2.10% ). The rates of increase stated are not indicative of historical increases awarded or a guarantee of future increases, but merely an appropriate assumption utilised in assessing DB liabilities. 4. This is the key assumption that determines assumed increases in pensions in payment and deferment in the UK only. Consistent with the derivation of the discount rate, the RPI assumption reflects the duration of the active liabilities to be adopted in the calculation of the future service obligations. This approach leads to a RPI assumption for the future service rate of 3.10% p.a. at reporting date (2017: 3.15% ; 2016: 2.90% ), as compared to the 2017 published assumption of 3.20% for both past service and future service. US pensions US other post-retirement benefits 2018 2017 2016 2018 2017 2016 % % % % % % Discount rate 1 4.00 4.25 4.25 4.00 4.25 4.25 Rate of increase in salaries 3.50 3.50 3.50 3.50 3.50 3.50 Initial healthcare cost trend rate n/a n/a n/a 7.50 7.00 7.50 Ultimate healthcare cost trend rate 2 n/a n/a n/a 4.50 4.50 4.50 1. The discount rates for pension liabilities have been determined by reference to appropriate yields on high-quality corporate bonds prevailing in the US debt markets at the reporting date with an expected duration of plan liabilities of 17 years . 2. The ultimate healthcare cost trend rate will reach the ultimate trend in 2026/28 (2017 and 2016: 2024/25). For sensitivity analysis see note 33. 2018 2017 2016 UK years US years UK years US years UK years US years Assumed life expectations for a retiree age 65 Males 22.3 22.0 22.9 21.9 22.8 21.8 Females 23.9 24.2 24.7 24.1 25.2 24.0 In 20 years: Males 23.7 23.6 25.1 23.6 25.1 23.5 Females 25.5 25.8 27.1 25.7 27.6 25.6
Amounts recognised in the statement of financial position	Amounts recognised in the statement of financial position 2018 2017 2016 £m £m £m Present value of funded obligations 1 (23,747 ) (25,890 ) (28,648 ) Fair value of plan assets 23,858 24,375 26,434 111 (1,515 ) (2,214 ) Present value of unfunded obligations (307 ) (340 ) (304 ) Other post-employment liabilities (67 ) (78 ) (67 ) Net defined benefit liability (263 ) (1,933 ) (2,585 ) Represented by: Liabilities (1,672 ) (2,536 ) (2,995 ) Assets 1,409 603 410 (263 ) (1,933 ) (2,585 ) The geographical split of pensions and other post-retirement benefits is as shown below: UK Pensions US Pensions US other post-retirement benefits 2018 2017 2016 2018 2017 2016 2018 2017 2016 £m £m £m £m £m £m £m £m £m Present value of funded obligations 1 (14,152 ) (15,565 ) (19,341 ) (6,349 ) (6,790 ) (5,916 ) (3,246 ) (3,535 ) (3,391 ) Fair value of plan assets 15,330 15,489 19,401 6,030 6,322 5,136 2,498 2,564 1,897 1,178 (76 ) 60 (319 ) (468 ) (780 ) (748 ) (971 ) (1,494 ) Present value of unfunded obligations (74 ) (80 ) (75 ) (233 ) (260 ) (229 ) — — — Other post-employment liabilities — — — — — — (67 ) (78 ) (67 ) Net defined benefit asset/(liability) 1,104 (156 ) (15 ) (552 ) (728 ) (1,009 ) (815 ) (1,049 ) (1,561 ) Represented by: Liabilities (74 ) (536 ) (300 ) (783 ) (951 ) (1,134 ) (815 ) (1,049 ) (1,561 ) Assets 1,178 380 285 231 223 125 — — — 1,104 (156 ) (15 ) (552 ) (728 ) (1,009 ) (815 ) (1,049 ) (1,561 ) 1. Present value of funded obligations split approximately as follows: • UK pensions at 31 March 2018 : 10% active members ( 2017 : 12% ; 2016 : 12% ); 18% deferred members ( 2017 : 19% ; 2016 : 18% ); 72% pensioner members ( 2017 : 69% ; 2016 : 70% ) • US pensions at 31 March 2018 : 38% active members (2017: 38% ; 2016 : 39% ); 8% deferred members ( 2017 : 9% ; 2016 : 9% ); 54% pensioner members ( 2017 : 53% ; 2016 : 52% ) • US other post-retirement benefits at 31 March 2018 : 38% active members ( 2017 : 39% ; 2016 : 41% ); 0% deferred members ( 2017 : 0% ; 2016 : 0% ); 62% pensioner members ( 2017 : 61% ; 2016 : 59% )
Amounts recognised in the income statement and statement of other comprehensive income	Amounts recognised in the income statement and statement of other comprehensive income 2018 2017 2016 £m £m £m Included within operating costs Administration costs 16 16 16 Included within payroll costs Defined benefit scheme costs: Current service cost 193 232 221 Past service cost – augmentations 1 1 3 Past service credit – redundancies (1 ) (1 ) (1 ) Special termination benefit cost – redundancies 9 7 11 202 239 234 Included within finance income and costs Net interest cost 65 105 112 Included within gain on disposal of discontinued operations Administration costs — 5 2 Disposal of UK Gas Distribution — 34 — — 39 2 Total included in income statement 1, 2 283 399 364 Remeasurement gains of pension assets and post-retirement benefit obligations 1,313 348 539 Exchange adjustments 175 (345 ) (81 ) Total included in the statement of other comprehensive income 2 1,488 3 458 1. Amounts recognised in the income statement include operating costs of £nil ( 2017 : £1 million ; 2016 : £1 million ); payroll costs of £nil ( 2017 : £35 million ; 2016 : £28 million ); and net interest of £nil ( 2017 : £2 million income; 2016 : £1 million cost) presented within profit from discontinued operations. These amounts all relate to UK pensions. 2. Amounts recognised in the statement of other comprehensive income include remeasurements of pension assets and post-retirement benefit obligations of £nil ( 2017 : £75 million loss; 2016 : £129 million gain) presented within discontinued operations. These amounts all relate to UK pensions. The geographical split of pensions and other post-retirement benefits is as shown below: UK Pensions US Pensions US other post-retirement benefits 2018 2017 2016 2018 2017 2016 2018 2017 2016 £m £m £m £m £m £m £m £m £m Included within operating costs Administration costs 6 6 9 9 9 6 1 1 1 Included within payroll costs Defined benefit scheme costs: Current service cost 49 76 74 98 103 95 46 53 52 Past service cost – augmentations 1 1 3 — — — — — — Past service credit – redundancies (1 ) (1 ) (1 ) — — — — — — Special termination benefit cost – redundancies 9 7 11 — — — — — — 58 83 87 98 103 95 46 53 52 Included within finance income and costs Net interest cost 3 — 18 27 43 36 35 62 58 Included within gain on disposal of discontinued operations Administration costs — 5 2 — — — — — — Disposal of UK Gas Distribution — 34 — — — — — — — — 39 2 — — — — — — Total included in income statement 67 128 116 134 155 137 82 116 111 Remeasurement gains/(losses) of pension assets and post retirement benefit obligations 1,177 (541 ) 534 27 319 (67 ) 109 570 72 Exchange adjustments — — — 75 (140 ) (33 ) 100 (205 ) (48 ) Total included in the statement of other comprehensive income 1,177 (541 ) 534 102 179 (100 ) 209 365 24
Disclosure of reconciliation of the net defined benefit liability	Reconciliation of the net defined benefit liability 2018 2017 2016 £m £m £m Opening net defined benefit liability (1,933 ) (2,585 ) (3,258 ) Cost recognised in the income statement (283 ) (399 ) (364 ) Remeasurement and foreign exchange effects recognised in the statement of other comprehensive income 1,488 3 458 Employer contributions 475 1,073 587 Other movements (10 ) (25 ) (8 ) Closing net defined benefit liability (263 ) (1,933 ) (2,585 ) The geographical split of pensions and other post-retirement benefits is as shown below: UK pensions US pensions US other post-retirement benefits 2018 2017 2016 2018 2017 2016 2018 2017 2016 £m £m £m £m £m £m £m £m £m Opening net defined benefit liability (156 ) (15 ) (672 ) (728 ) (1,009 ) (1,003 ) (1,049 ) (1,561 ) (1,583 ) Cost recognised in the income statement (67 ) (128 ) (116 ) (134 ) (155 ) (137 ) (82 ) (116 ) (111 ) Remeasurement and foreign exchange effects recognised in the statement of other comprehensive income 1,177 (541 ) 534 102 179 (100 ) 209 365 24 Employer contributions 150 528 239 208 257 231 117 288 117 Other movements — — — — — — (10 ) (25 ) (8 ) Closing net defined benefit 1,104 (156 ) (15 ) (552 ) (728 ) (1,009 ) (815 ) (1,049 ) (1,561 ) Changes in the present value of defined benefit obligations (including unfunded obligations) 2018 2017 2016 £m £m £m Opening defined benefit obligations (26,230 ) (28,952 ) (29,592 ) Current service cost (193 ) (232 ) (221 ) Interest cost (775 ) (1,057 ) (1,026 ) Actuarial (losses)/gains – experience (100 ) 166 659 Actuarial gains – demographic assumptions 671 225 — Actuarial gains/(losses) – financial assumptions 174 (3,377 ) 218 Past service credit – redundancies 1 1 1 Special termination benefit cost – redundancies (9 ) (7 ) (11 ) Past service cost – augmentations (1 ) (1 ) (3 ) Medicare subsidy received (21 ) (14 ) (15 ) Obligations transferred on disposal of UK Gas Distribution — 6,970 — Employee contributions (1 ) (1 ) (2 ) Benefits paid 1,285 1,443 1,348 Exchange adjustments 1,145 (1,394 ) (308 ) Closing defined benefit obligations (24,054 ) (26,230 ) (28,952 ) The geographical split of pensions and other post-retirement benefits is as shown below: UK pensions US pensions US other post-retirement benefits 2018 2017 2016 2018 2017 2016 2018 2017 2016 £m £m £m £m £m £m £m £m £m Opening defined benefit obligations (15,645 ) (19,416 ) (20,125 ) (7,050 ) (6,145 ) (6,055 ) (3,535 ) (3,391 ) (3,412 ) Current service cost (49 ) (76 ) (74 ) (98 ) (103 ) (95 ) (46 ) (53 ) (52 ) Interest cost (366 ) (615 ) (649 ) (273 ) (285 ) (242 ) (136 ) (157 ) (135 ) Actuarial (losses)/gains – experience (95 ) 106 552 (38 ) (2 ) 15 33 62 92 Actuarial gains – demographic assumptions 565 214 — 30 2 — 76 9 — Actuarial gains/(losses) – financial assumptions 604 (3,751 ) — (279 ) 37 120 (151 ) 337 98 Past service credit – redundancies 1 1 1 — — — — — — Special termination benefit cost – redundancies (9 ) (7 ) (11 ) — — — — — — Past service cost – augmentations (1 ) (1 ) (3 ) — — — — — — Medicare subsidy received — — — — — — (21 ) (14 ) (15 ) Obligations transferred on disposal of UK Gas Distribution — 6,970 — — — — — — — Employee contributions (1 ) (1 ) (2 ) — — — — — — Benefits paid 770 931 895 362 349 310 153 163 143 Exchange adjustments — — — 764 (903 ) (198 ) 381 (491 ) (110 ) Closing defined benefit obligations (14,226 ) (15,645 ) (19,416 ) (6,582 ) (7,050 ) (6,145 ) (3,246 ) (3,535 ) (3,391 ) 23. Pensions and other post-retirement benefits continued Changes in the value of plan assets 2018 2017 2016 £m £m £m Opening fair value of plan assets 24,375 26,434 26,408 Interest income 710 952 914 Return on plan assets in excess of/(less than) interest 568 3,334 (338 ) Administration costs (16 ) (21 ) (18 ) Employer contributions 475 1,073 587 Employee contributions 1 1 2 Benefits paid (1,285 ) (1,443 ) (1,348 ) Exchange adjustments (970 ) 1,049 227 Assets transferred on disposal of UK Gas Distribution — (7,004 ) — Closing fair value of plan assets 23,858 24,375 26,434 Actual return on plan assets 1,278 4,286 576 Expected contributions to plans in the following year 363 491 686 The geographical split of pensions and other post-retirement benefits is as shown below: UK pensions US pensions US other post-retirement benefits 2018 2017 2016 2018 2017 2016 2018 2017 2016 £m £m £m £m £m £m £m £m £m Opening fair value of plan assets 15,489 19,401 19,453 6,322 5,136 5,052 2,564 1,897 1,903 Interest income 363 615 631 246 242 206 101 95 77 Return on plan assets in excess 103 2,890 (18 ) 314 282 (202 ) 151 162 (118 ) Administration costs (6 ) (11 ) (11 ) (9 ) (9 ) (6 ) (1 ) (1 ) (1 ) Employer contributions 150 528 239 208 257 231 117 288 117 Employee contributions 1 1 2 — — — — — — Benefits paid (770 ) (931 ) (895 ) (362 ) (349 ) (310 ) (153 ) (163 ) (143 ) Exchange adjustments — — — (689 ) 763 165 (281 ) 286 62 Assets transferred on disposal of UK Gas Distribution — (7,004 ) — — — — — — — Closing fair value of plan assets 15,330 15,489 19,401 6,030 6,322 5,136 2,498 2,564 1,897 Actual return on plan assets 466 3,505 613 560 524 4 252 257 (41 ) Expected contributions to plans in the following year 140 128 228 221 229 220 2 134 238
Disclosure of asset allocations	Each plan’s investment strategy is formulated specifically in order to target specific asset allocations, asset returns and to manage risk. The asset allocation of the plans as at 31 March 2018 is as follows: UK pensions 1 US pensions 2 US other post-retirement benefits 3 % % % Equities 14.6 42.0 60.9 Corporate bonds 25.8 24.8 1.0 Government securities 36.7 16.2 20.4 Property 6.3 4.6 — Diversified alternatives 5.1 10.3 12.2 Liability matching assets 7.7 — — Other 3.8 2.1 5.5 100.0 100.0 100.0 1. Of total assets at year-end date, 57.8% is invested in the UK, 18.4% in the US and 23.7% in other countries, including the EU. 2. Of total assets at year-end date, 88.5% is invested in the US, 1.2% in the UK and 10.3% in other countries. 3. Of total assets at year-end date, 91.8% is invested within the US, 0.2% in the UK and 8.0% in other countries. Asset allocations Within the asset allocations below there is significant diversification across regions, asset managers, currencies and bond categories. UK pensions 2018 2017 2016 Quoted Unquoted Total Quoted Unquoted Total Quoted Unquoted Total £m £m £m £m £m £m £m £m £m Equities 1 1,420 813 2,233 2,624 596 3,220 3,272 962 4,234 Corporate bonds 2 3,949 — 3,949 3,526 — 3,526 5,601 — 5,601 Government securities 5,629 — 5,629 5,406 — 5,406 6,059 — 6,059 Property 129 834 963 90 708 798 90 1,081 1,171 Diversified alternatives 3 99 690 789 250 628 878 159 505 664 Liability matching assets 4 1,174 — 1,174 1,162 — 1,162 1,020 — 1,020 Other 5 211 382 593 63 436 499 649 3 652 12,611 2,719 15,330 13,121 2,368 15,489 16,850 2,551 19,401 1. Included within equities at 31 March 2018 were ordinary shares of National Grid plc with a value of £nil ( 2017 : £2 million ; 2016 : £7 million ). 2. Included within corporate bonds at 31 March 2018 was an investment in a number of bonds issued by subsidiary undertakings with a value of £nil ( 2017 : £nil ; 2016 : £70 million ). 3. Includes return seeking non-conventional asset classes which include hedge funds, private debt, mezzanine debt, infrastructure debt and fixed income debt instruments. 4. Includes liability-driven investment vehicles. 5. Includes cash and cash type instruments. US pensions 2018 2017 2016 Quoted Unquoted Total Quoted Unquoted Total Quoted Unquoted Total £m £m £m £m £m £m £m £m £m Equities 577 1,954 2,531 698 1,915 2,613 625 1,508 2,133 Corporate bonds 1,085 413 1,498 1,130 537 1,667 954 483 1,437 Government securities 414 565 979 872 71 943 711 — 711 Property — 279 279 — 335 335 — 276 276 Diversified alternatives 1 198 421 619 209 433 642 163 334 497 Other 2 20 104 124 31 91 122 — 82 82 2,294 3,736 6,030 2,940 3,382 6,322 2,453 2,683 5,136 1. Includes return seeking non-conventional asset classes which include hedge funds, real estate debt and limited partnerships. 2. Includes collateral, liquid investments and cash deposits, debtors and creditors. US other post-retirement benefits 2018 2017 2016 Quoted Unquoted Total Quoted Unquoted Total Quoted Unquoted Total £m £m £m £m £m £m £m £m £m Equities 412 1,110 1,522 405 1,162 1,567 281 853 1,134 Corporate bonds 24 — 24 19 — 19 37 1 38 Government securities 508 2 510 520 1 521 390 — 390 Diversified alternatives 1 161 144 305 166 149 315 122 104 226 Other 2 — 137 137 — 142 142 — 109 109 1,105 1,393 2,498 1,110 1,454 2,564 830 1,067 1,897 1. Includes return seeking non-conventional asset classes which include hedge funds, real estate debt and limited partnerships. 2. Includes collateral, liquid investments and cash deposits, debtors and creditors.

Provisions (Tables)

Provisions (Tables)	12 Months Ended
	Mar. 31, 2018
Other Provisions, Contingent Liabilities And Contingent Assets [Abstract]
Schedule of provisions	Environmental £m Decommissioning £m Restructuring £m Emissions £m Other £m Total provisions £m At 1 April 2016 1,169 141 30 18 361 1,719 Exchange adjustments 137 13 — 2 36 188 Additions 572 107 16 12 308 1,015 Unused amounts reversed (75 ) — (5 ) — (6 ) (86 ) Unwinding of discount 60 — 1 — 12 73 Utilised (110 ) (19 ) (25 ) — (73 ) (227 ) Disposal of UK Gas Distribution (32 ) (21 ) — — (41 ) (94 ) At 31 March 2017 1,721 221 17 32 597 2,588 Exchange adjustments (158 ) (9 ) — (2 ) (26 ) (195 ) Additions 27 2 10 12 23 74 Unused amounts reversed 1 (45 ) (19 ) (6 ) — (37 ) (107 ) Unwinding of discount 61 5 1 — 8 75 Utilised 2 (75 ) (6 ) (7 ) (34 ) (146 ) (268 ) Transfers 3 — — (12 ) — (103 ) (115 ) At 31 March 2018 1,531 194 3 8 316 2,052 2018 2017 £m £m Current 273 416 Non-current 1,779 2,172 2,052 2,588 1. Unused amounts reversed from other provisions include £16 million in relation to discontinued operations. 2. Utilised amounts for other provisions include £77 million in relation to discontinued operations. 3. Represents net amounts transferred to trade and other payables (see note 21) of £115 million ( 2017 : £nil ).
Schedule of environmental provision	The environmental provision is as follows: 2018 2017 Discounted £m Undiscounted £m Real discount rate Discounted £m Undiscounted £m Real discount rate UK sites 213 235 1 % 242 270 1 % US sites 1,318 1,410 1 % 1,479 1,619 1 % 1,531 1,645 1,721 1,889

Share capital (Tables)

Share capital (Tables)	12 Months Ended
	Mar. 31, 2018
Share Capital, Reserves And Other Equity Interest [Abstract]
Schedule of share capital	An equity instrument is any contract that includes a residual interest in the consolidated assets of the Company after deducting all its liabilities and is recorded at the proceeds received, net of direct issue costs, with an amount equal to the nominal amount of the shares issued included in the share capital account and the balance recorded in the share premium account. Allotted, called up and fully paid million £m At 1 April 2016 3,924 447 Issued during the year in lieu of dividends 1 19 2 At 31 March 2017 3,943 449 Effect of share consolidation 2 (328 ) — Issued during the year in lieu of dividends 1 23 3 At 31 March 2018 3,638 452 1. The issue of shares under the scrip dividend programme is considered to be a bonus issue under the terms of the Companies Act 2006 and the nominal value of the shares is charged to the share premium account. 2. On 22 May 2017 the ordinary share capital was consolidated with 11 new ordinary shares of 12 204 / 473 pence nominal value issued for every 12 existing ordinary shares of 11 17 / 43 pence nominal value. This consolidation was completed to maintain the comparability of the Company’s share price before and after the special dividend.

Other equity reserves (Tables)

Other equity reserves (Tables)	12 Months Ended
	Mar. 31, 2018
Disclosure of reserves within equity [abstract]
Schedule of reserves within equity	As the amounts included in other equity reserves are not attributable to any of the other classes of equity presented, they have been disclosed as a separate classification of equity. Translation £m Cash flow hedge £m Available- for-sale £m Capital redemption £m Merger £m Total £m At 1 April 2015 479 (109 ) 94 19 (5,165 ) (4,682 ) Exchange adjustments 1 69 — — — — 69 Net gains taken to equity — 50 43 — — 93 Transferred to profit or loss — 29 — — — 29 Tax — (15 ) (17 ) — — (32 ) At 31 March 2016 548 (45 ) 120 19 (5,165 ) (4,523 ) Exchange adjustments 1 346 — — — — 346 Net (losses)/gains taken to equity — (36 ) 81 — — 45 Transferred to/(from) profit or loss — 227 (25 ) — — 202 Tax — (43 ) (14 ) — — (57 ) At 31 March 2017 894 103 162 19 (5,165 ) (3,987 ) Exchange adjustments 1 (504 ) — — — — (504 ) Net gains/(losses) taken to equity — 19 (30 ) — — (11 ) Share of net gains taken to equity of associates — 5 — — — 5 Transferred from profit or loss — (3 ) (73 ) — — (76 ) Tax — 4 29 — — 33 At 31 March 2018 390 128 88 19 (5,165 ) (4,540 ) 1. The exchange adjustments recorded in the translation reserve comprise a loss of £1,304 million ( 2017 : gain of £1,364 million ; 2016 : gain of £275 million ) relating to the translation of foreign operations offset by a gain of £800 million ( 2017 : loss of £1,018 million ; 2016 : loss of £206 million ) relating to borrowings, cross-currency swaps and foreign exchange forward contracts used to hedge the net investment in the non-sterling denominated subsidiaries.

Net debt (Tables)

Net debt (Tables)	12 Months Ended
	Mar. 31, 2018
Subclassifications of assets, liabilities and equities [abstract]
Reconciliation of net cash flow to movement in net debt	Reconciliation of net cash flow to movement in net debt 2018 2017 2016 £m £m £m (Decrease)/increase in cash and cash equivalents (807 ) 984 4 (Decrease)/increase in financial investments (5,953 ) 5,675 391 Decrease/(increase) in borrowings and related derivatives 1 1,209 (3,715 ) (1,100 ) Net interest paid on the components of net debt 2 808 1,955 810 Change in debt resulting from cash flows (4,743 ) 4,899 105 Changes in fair value of financial assets and liabilities and exchange movements 2,098 (2,273 ) (515 ) Net interest charge on the components of net debt (1,017 ) (2,401 ) (913 ) Disposal of UK Gas Distribution — 5,890 — Other non-cash movements (66 ) (64 ) (87 ) Movement in net debt (net of related derivative financial instruments) in the year (3,728 ) 6,051 (1,410 ) Net debt (net of related derivative financial instruments) at start of year (19,274 ) (25,325 ) (23,915 ) Net debt (net of related derivative financial instruments) at end of year (23,002 ) (19,274 ) (25,325 )
Schedule of components of net debt	Net debt is made up as follows: 2018 2017 2016 £m £m £m Cash, cash equivalents and financial investments 3,023 9,880 3,125 Borrowings and bank overdrafts (26,625 ) (28,638 ) (28,344 ) Financing derivatives 1 600 (516 ) (106 ) (23,002 ) (19,274 ) (25,325 ) 1. The financing derivatives balance included in net debt excludes the commodity derivatives and the Further Acquisition Agreement (FAA) derivative (see note 16). Included within the financing derivatives balance is £12 million ( 2017 : £18 million ; 2016 : £(16) million ) of derivative cash flows in relation to capital expenditure. This is included within investing activities and not financing activities in the cash flow statement. 2. Excludes £27 million ( 2017 : £nil ; 2016 : £nil ) cash interest from the Quadgas HoldCo Limited shareholder loan included within interest received in the cash flow statement.
Schedule of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments	Analysis of changes in net debt Cash and cash equivalents £m Bank overdrafts £m Net cash and cash equivalents £m Financial investments £m Borrowings £m Financing derivatives £m Total 1 £m At 1 April 2016 127 (3 ) 124 2,998 (28,341 ) (106 ) (25,325 ) Cash flow 1,001 (17 ) 984 5,624 (2,196 ) 487 4,899 Fair value gains and losses and exchange movements 16 — 16 141 (1,527 ) (903 ) (2,273 ) Interest income/(charges) 2 — — — 53 (2,221 ) (233 ) (2,401 ) Other non-cash movements — — — — (294 ) 230 (64 ) Disposal (5 ) 20 15 (75 ) 5,941 9 5,890 At 31 March 2017 1,139 — 1,139 8,741 (28,638 ) (516 ) (19,274 ) Cash flow 2 (807 ) — (807 ) (5,983 ) 2,108 (61 ) (4,743 ) Fair value gains and losses and exchange movements (3 ) — (3 ) (149 ) 1,088 1,162 2,098 Interest income/(charges) 2 — — — 85 (1,117 ) 15 (1,017 ) Other non-cash movements — — — — (66 ) — (66 ) At 31 March 2018 329 — 329 2,694 (26,625 ) 600 (23,002 ) Balances at 31 March 2018 comprise: Non-current assets — — — — — 1,170 1,170 Current assets 329 — 329 2,694 — 375 3,398 Current liabilities — — — — (4,447 ) (620 ) (5,067 ) Non-current liabilities — — — — (22,178 ) (325 ) (22,503 ) 329 — 329 2,694 (26,625 ) 600 (23,002 ) 1. Includes accrued interest at 31 March 2018 of £197 million ( 2017 : £210 million ; 2016 : £243 million ). 2. An exceptional income of £3 million ( 2017 : £1,313 million expense; 2016 : £nil ) is included in net interest charge on the components of net debt and an exceptional cash outflow of £231 million ( 2017 : £1,052 million ; 2016 : £nil ) is included in cash flow on the components of net debt.

Commitments and contingencies (

Commitments and contingencies (Tables)	12 Months Ended
	Mar. 31, 2018
Other Provisions, Contingent Liabilities And Contingent Assets [Abstract]
Schedule of capital commitments and contingent liabilities	2018 2017 £m £m Future capital expenditure Contracted for but not provided 1 1,843 1,913 Operating lease commitments Less than 1 year 68 95 In 1 to 2 years 44 82 In 2 to 3 years 40 58 In 3 to 4 years 37 56 In 4 to 5 years 35 54 More than 5 years 219 274 443 619 Energy purchase commitments 2 Less than 1 year 1,237 1,325 In 1 to 2 years 700 744 In 2 to 3 years 563 587 In 3 to 4 years 449 507 In 4 to 5 years 410 436 More than 5 years 1,969 2,100 5,328 5,699 Guarantees Guarantee of sublease for US property (expires 2040) 178 225 Guarantees of certain obligations of Grain LNG Import Terminal (expire up to 2028) 46 100 Guarantees of certain obligations for construction of HVDC West Coast Link (expires 2018) 213 281 Guarantees of certain obligations of Nemo Link Limited (various expiry dates) 63 140 Guarantees of certain obligations of National Grid North Sea Link Limited (various expiry dates) 3 1,009 1,059 Guarantees of certain obligations of St William Homes LLP (various expiry dates) 4 98 147 Guarantees of certain obligations for construction of IFA 2 SAS (expected expiry 2021) 3 729 354 Other guarantees and letters of credit (various expiry dates) 333 474 2,669 2,780 1. Following a review in the year, the basis on which we disclose capital commitments has been refined. 2. Energy purchase commitments relate to contractual commitments to purchase electricity or gas that are used to satisfy physical delivery requirements to our customers or for energy that we use ourselves (i.e. normal purchase, sale or usage) and hence are accounted for as ordinary purchase contracts. Details of commodity contract derivatives that do not meet the normal purchase, sale or usage criteria, and hence are accounted for as derivative contracts, are shown in note 16(b). 3. Included within total guarantees are guarantees to both joint ventures and EPC contractors regarding the construction of interconnectors of £739 million (2017: £555 million ). 4. Includes guarantees to related parties.

Related party transactions (Tab

Related party transactions (Tables)	12 Months Ended
	Mar. 31, 2018
Related party transactions [abstract]
Schedule of transactions between related parties	The following significant transactions with related parties were in the normal course of business. Amounts receivable from and payable to related parties are due on normal commercial terms: 2018 2017 2016 £m £m £m Sales: Goods and services supplied to a pension plan 3 3 3 Sales: Goods and services supplied to joint ventures 1 14 78 9 Sales: Goods and services supplied to associates 2 220 — 4 Purchases: Goods and services received from joint ventures 3 135 168 183 Purchases: Goods and services received from associates 3 160 169 83 Receivable from joint ventures 4 160 64 7 Receivable from associates 4 376 457 — Payable to joint ventures 5 — 84 96 Payable to associates 17 27 7 Interest income from joint ventures 4 — — Interest income from associates 27 — — Dividends received from joint ventures 6 43 75 48 Dividends received from associates 7 170 24 24 1. In 2018 £5 million (2017: £68 million ) of property sites were sold to joint venture St William Homes LLP. 2. Sales in the year relate to transactions with Quadgas HoldCo Limited. Within this is other income of £54 million relating to a Transitional Service Agreement following the sale of the UK Gas Distribution business to Quadgas HoldCo Limited. 3. During the year the Group received goods and services from a number of US associates, both for the transportation of gas and for pipeline services in the US. Additionally, goods and services were received from UK joint ventures for the construction of a transmission link in the UK. 4. Amounts receivable from associates includes a loan receivable balance from Quadgas HoldCo Limited of £352 million (2017: £434 million ) and a loan receivable balance of £130 million (2017: £61 million ) from Nemo Link Limited (a joint venture). 5. In previous years the amounts payable to joint ventures include deposits received for National Grid property sites from St William Homes LLP which have been settled during the year. 6. Dividends in respect of joint ventures were received from BritNed Development Limited. 7. Within dividends received from associates in 2018, £144 million (2017: £nil ) was from Quadgas HoldCo Limited.

Financial risk management (Tabl

Financial risk management (Tables)	12 Months Ended
	Mar. 31, 2018
Financial Instruments [Abstract]
Disclosure of treasury credit risk	As at 31 March 2018, the following limits were in place for investments held with banks and financial institutions: Maximum limit Long-term limit Triple ‘A’ G7 sovereign entities (AAA) 1,853 927 Triple ‘A’ vehicles (AAA) 500 — Triple ‘A’ range institutions and non G7 sovereign entities (AAA) 1,011 506 Double ‘A+’ G7 sovereign entities (AA+) 1,685 843 Double ‘A’ range institutions (AA) 674 to 843 337 to 421 Single ‘A’ range institutions (A) 236 to 337 118 to 169
Disclosure of offsetting of financial assets and financial liabilities	National Grid has similar arrangements in relation to bank account balances and bank overdrafts, and trade payables and trade receivables which are subject to general terms and conditions. However, these balances are immaterial. Related amounts At 31 March 2018 Gross £m Gross amounts offset £m Net amount £m Financial instruments £m Cash £m Net amount £m Assets Financing derivatives 1,545 — 1,545 (523 ) (772 ) 250 Commodity contract derivatives 69 — 69 (16 ) — 53 Further Acquisition Agreement derivative 1 110 — 110 — — 110 1,724 — 1,724 (539 ) (772 ) 413 Liabilities Financing derivatives (945 ) — (945 ) 523 326 (96 ) Commodity contract derivatives (116 ) — (116 ) 16 7 (93 ) (1,061 ) — (1,061 ) 539 333 (189 ) 663 — 663 — (439 ) 224 1. The Group held a put/call option as at 31 March 2017. The fair value was £nil. Related amounts At 31 March 2017 Gross carrying amounts £m Gross amounts offset £m Net amount presented in statement of financial position £m Financial instruments £m Cash collateral received/ pledged £m Net amount £m Assets Financing derivatives 1,707 — 1,707 (718 ) (692 ) 297 Commodity contract derivatives 106 — 106 (25 ) — 81 1,813 — 1,813 (743 ) (692 ) 378 Liabilities Financing derivatives (2,223 ) — (2,223 ) 718 1,230 (275 ) Commodity contract derivatives (170 ) — (170 ) 25 18 (127 ) (2,393 ) — (2,393 ) 743 1,248 (402 ) (580 ) — (580 ) — 556 (24 )
Disclosure of maturity analysis for financial liabilities	The following is an analysis of the contractual undiscounted cash flows payable under financial liabilities and derivative assets and liabilities as at the reporting date: At 31 March 2018 Less than £m 1 to 2 £m 2 to 3 £m More than £m Total £m Non-derivative financial liabilities Borrowings, excluding finance lease liabilities (4,099 ) (1,642 ) (2,325 ) (18,023 ) (26,089 ) Interest payments on borrowings 1 (730 ) (692 ) (629 ) (12,897 ) (14,948 ) Finance lease liabilities (60 ) (60 ) (45 ) (148 ) (313 ) Other non interest-bearing liabilities (2,840 ) (359 ) — — (3,199 ) Derivative financial liabilities Derivative contracts – receipts 1,069 601 130 250 2,050 Derivative contracts – payments (890 ) (263 ) (188 ) (529 ) (1,870 ) Commodity contract derivatives (80 ) (33 ) (26 ) 1 (138 ) (7,630 ) (2,448 ) (3,083 ) (31,346 ) (44,507 ) At 31 March 2017 Less than 1 year £m 1 to 2 years £m 2 to 3 years £m More than 3 years £m Total £m Non-derivative financial liabilities Borrowings, excluding finance lease liabilities (5,142 ) (1,864 ) (1,750 ) (19,245 ) (28,001 ) Interest payments on borrowings 1 (767 ) (707 ) (670 ) (12,975 ) (15,119 ) Finance lease liabilities (66 ) (58 ) (61 ) (183 ) (368 ) Other non interest-bearing liabilities (2,989 ) (260 ) — — (3,249 ) Derivative financial liabilities Derivative contracts – receipts 571 961 572 234 2,338 Derivative contracts – payments (1,551 ) (959 ) (304 ) (610 ) (3,424 ) Commodity contract derivatives (15 ) (18 ) (8 ) — (41 ) (9,959 ) (2,905 ) (2,221 ) (32,779 ) (47,864 ) 1. The interest on borrowings is calculated based on borrowings held at 31 March without taking account of future issues. Floating rate interest is estimated using a forward interest rate curve as at 31 March. Payments are included on the basis of the earliest date on which the Company can be required to settle.
Disclosure of financial instruments by type of interest rate	During 2018 and 2017, net debt was managed using derivative financial instruments to hedge interest rate risk as follows: 2018 2017 Fixed rate £m Floating rate £m Inflation linked £m Other 1 £m Total £m Fixed rate £m Floating rate £m Inflation linked £m Other 1 £m Total £m Cash and cash equivalents — 302 — 27 329 940 199 — — 1,139 Financial investments 31 2,625 — 38 2,694 44 8,691 — 6 8,741 Borrowings (16,144 ) (3,191 ) (7,290 ) — (26,625 ) (17,681 ) (3,917 ) (7,040 ) — (28,638 ) Pre-derivative position (16,113 ) (264 ) (7,290 ) 65 (23,602 ) (16,697 ) 4,973 (7,040 ) 6 (18,758 ) Derivative effect 1,735 (1,237 ) 102 — 600 1,424 (1,785 ) (155 ) — (516 ) Net debt position (14,378 ) (1,501 ) (7,188 ) 65 (23,002 ) (15,273 ) 3,188 (7,195 ) 6 (19,274 ) 1. Represents financial instruments which are not directly affected by interest rate risk, such as investments in equity or other similar financial instruments.
Disclosure of nature and extent of risks arising from financial instruments	The currency exposure on other financial instruments is as follows: 2018 2017 Sterling £m Euro £m Dollar £m Other £m Total £m Sterling £m Euro £m Dollar £m Other £m Total £m Trade and other receivables 253 — 1,528 — 1,781 83 — 1,660 — 1,743 Trade and other payables (1,124 ) — (1,793 ) — (2,917 ) (1,209 ) — (1,795 ) — (3,004 ) Other non-current liabilities (144 ) — (254 ) — (398 ) (100 ) — (315 ) — (415 ) During 2018 and 2017, derivative financial instruments were used to manage foreign currency risk as follows: 2018 2017 Sterling £m Euro £m Dollar £m Other £m Total £m Sterling £m Euro £m Dollar £m Other £m Total £m Cash and cash equivalents 294 2 33 — 329 1,110 — 29 — 1,139 Financial investments 1,471 69 1,125 29 2,694 6,824 98 1,753 66 8,741 Borrowings (10,912 ) (3,794 ) (11,068 ) (851 ) (26,625 ) (11,099 ) (5,373 ) (10,729 ) (1,437 ) (28,638 ) Pre-derivative position (9,147 ) (3,723 ) (9,910 ) (822 ) (23,602 ) (3,165 ) (5,275 ) (8,947 ) (1,371 ) (18,758 ) Derivative effect 3,748 3,793 (7,992 ) 1,051 600 2,310 6,241 (10,708 ) 1,641 (516 ) Net debt position (5,399 ) 70 (17,902 ) 229 (23,002 ) (855 ) 966 (19,655 ) 270 (19,274 ) 2018 2017 Income statement £m Other equity reserves £m Income statement £m Other equity reserves £m Financial risk (post-tax) 1 : UK RPI change of 0.5% 2 30 — 28 — UK interest rates change of 0.5% 43 26 64 35 US interest rates change of 0.5% 39 17 61 22 US dollar exchange rate change of 10% 3 48 479 46 604 1. Excludes sensitivities to the Further Acquisition Agreement derivative. Further details on sensitivities are provided in note 30(f). 2. Excludes sensitivities to LPI curve. Further details on sensitivities are provided in note 30(f). 3. The other equity reserves impact does not reflect the exchange translation in our US subsidiaries’ net assets. It is estimated this would change by £1,040 million ( 2017 : £988 million ) in the opposite direction if the dollar exchange rate changed by 10% . Additional sensitivities in respect to commodity price risk and to our derivative fair values are as follows: 2018 2017 Income statement £m Net assets £m Income statement £m Net assets £m Commodity risk 1 (post-tax): 10% increase in commodity prices 23 23 28 28 10% decrease in commodity prices (23 ) (23 ) (29 ) (29 ) Assets and liabilities carried at fair value (pre-tax) 2 : 10% fair value change in derivative financial instruments 3 60 60 (52 ) (52 ) 10% fair value change in commodity contract derivative liabilities (5 ) (5 ) (6 ) (6 ) 1. Represents potential impact on fair values of commodity contract derivatives only. 2. Excludes sensitivities to the Further Acquisition Agreement derivative. Further details on sensitivities are provided in note 30(f). 3. The effect of a 10% change in fair value assumes no hedge accounting. These estimates are those that have a significant risk of resulting in a material adjustment to the carrying values of assets and liabilities in the next year. 2018 2017 Income statement £m Net assets £m Income statement £m Net assets £m Pensions and other post-retirement benefits 1 (pre-tax): UK discount rate change of 0.5% 2 8 1,075 9 1,305 US discount rate change of 0.5% 2 15 623 17 669 UK RPI rate change of 0.5% 3 5 965 8 1,114 UK long-term rate of increase in salaries change of 0.5% 4 — 61 2 80 US long-term rate of increase in salaries change of 0.5% 3 44 3 51 UK change of one year to life expectancy at age 65 2 588 2 673 US change of one year to life expectancy at age 65 4 359 4 365 Assumed US healthcare cost trend rates change of 1% 31 448 37 510 Environmental provision: 10% change in estimated future cash flows 154 154 175 175 0.5% change in discount rate 56 56 67 67 1. The changes shown are a change in the annual pension or other post-retirement benefit service charge and change in the defined benefit obligations. 2. A change in the discount rate is likely to occur as a result of changes in bond yields and as such would be expected to be offset to a significant degree by a change in the value of the bond assets held by the plans. 3. The projected impact resulting from a change in RPI reflects the underlying effect on pensions in payment, pensions in deferment and resultant increases in salary assumptions. 4. This change has been applied to both the pre 1 April 2014 and post 1 April 2014 rate of increase in salary assumption.
Disclosure of fair value measurement of assets	The changes in value of our level 3 derivative financial instruments are as follows: Financing derivatives/ Further Acquisition Agreement derivative Commodity contract derivatives Investments in associates and available-for-sale investments Total 2018 2017 2018 2017 2018 2017 2018 2017 £m £m £m £m £m £m £m £m At 1 April (465 ) (196 ) (16 ) (27 ) 46 — (435 ) (223 ) Net gains/(losses) for the year 1,2 125 (35 ) 8 (2 ) — — 133 (37 ) Purchases — — 27 15 41 46 68 61 Settlements 231 — (20 ) (2 ) (3 ) — 208 (2 ) Reclassification into level 3 — (234 ) — — — — — (234 ) At 31 March (109 ) (465 ) (1 ) (16 ) 84 46 (26 ) (435 ) 1. Gain of £125 million ( 2017 : £35 million loss) is attributable to derivative financial instruments held at the end of the reporting period and has been recognised in finance costs in the income statement. 2. Gain of £35 million ( 2017 : £21 million loss) is attributable to commodity contract derivative financial instruments held at the end of the reporting period. 2018 2017 Level 1 £m Level 2 £m Level 3 £m Total £m Level 1 £m Level 2 £m Level 3 £m Total £m Assets Investments – available-for-sale 2,406 310 5 2,721 7,717 315 5 8,037 Investments – joint ventures and associates — — 79 79 — — 41 41 Financing derivatives — 1,544 1 1,545 — 1,692 15 1,707 Commodity contract derivatives — 8 61 69 — 22 84 106 Further Acquisition Agreement derivative — — 110 110 — — — — 2,406 1,862 256 4,524 7,717 2,029 145 9,891 Liabilities Financing derivatives — (725 ) (220 ) (945 ) — (1,743 ) (480 ) (2,223 ) Commodity contract derivatives — (54 ) (62 ) (116 ) — (70 ) (100 ) (170 ) — (779 ) (282 ) (1,061 ) — (1,813 ) (580 ) (2,393 ) 2,406 1,083 (26 ) 3,463 7,717 216 (435 ) 7,498
Disclosure of fair value measurement of liabilities	2018 2017 Level 1 £m Level 2 £m Level 3 £m Total £m Level 1 £m Level 2 £m Level 3 £m Total £m Assets Investments – available-for-sale 2,406 310 5 2,721 7,717 315 5 8,037 Investments – joint ventures and associates — — 79 79 — — 41 41 Financing derivatives — 1,544 1 1,545 — 1,692 15 1,707 Commodity contract derivatives — 8 61 69 — 22 84 106 Further Acquisition Agreement derivative — — 110 110 — — — — 2,406 1,862 256 4,524 7,717 2,029 145 9,891 Liabilities Financing derivatives — (725 ) (220 ) (945 ) — (1,743 ) (480 ) (2,223 ) Commodity contract derivatives — (54 ) (62 ) (116 ) — (70 ) (100 ) (170 ) — (779 ) (282 ) (1,061 ) — (1,813 ) (580 ) (2,393 ) 2,406 1,083 (26 ) 3,463 7,717 216 (435 ) 7,498 The changes in value of our level 3 derivative financial instruments are as follows: Financing derivatives/ Further Acquisition Agreement derivative Commodity contract derivatives Investments in associates and available-for-sale investments Total 2018 2017 2018 2017 2018 2017 2018 2017 £m £m £m £m £m £m £m £m At 1 April (465 ) (196 ) (16 ) (27 ) 46 — (435 ) (223 ) Net gains/(losses) for the year 1,2 125 (35 ) 8 (2 ) — — 133 (37 ) Purchases — — 27 15 41 46 68 61 Settlements 231 — (20 ) (2 ) (3 ) — 208 (2 ) Reclassification into level 3 — (234 ) — — — — — (234 ) At 31 March (109 ) (465 ) (1 ) (16 ) 84 46 (26 ) (435 ) 1. Gain of £125 million ( 2017 : £35 million loss) is attributable to derivative financial instruments held at the end of the reporting period and has been recognised in finance costs in the income statement. 2. Gain of £35 million ( 2017 : £21 million loss) is attributable to commodity contract derivative financial instruments held at the end of the reporting period.
Sensitivity analysis for types of market risk	The impacts on a post-tax basis of reasonably possible changes in significant level 3 assumptions are as follows: Financing derivatives Commodity contract derivatives 2018 2017 2018 2017 £m £m £m £m 10% increase in commodity prices 1 — — (1 ) 1 10% decrease in commodity prices 1 — — 3 — Volume forecast uplift 2 — — — (1 ) Volume forecast reduction 2 — — — 1 +10% market area price change — — (6 ) (13 ) -10% market area price change — — 5 9 +20 basis points change in Limited Price Inflation (LPI) market curve 3 (84 ) (93 ) — — -20 basis points change in LPI market curve 3 82 88 — — 1. Level 3 commodity price sensitivity is included within the sensitivity analysis disclosed in note 33. 2. Volumes were flexed using maximum and minimum historical averages, or by > 10% where historical averages were not available. 3. A reasonably possible change in assumption of other level 3 derivative financial instruments is unlikely to result in a material change in fair values.

Borrowing facilities (Tables)

Borrowing facilities (Tables)	12 Months Ended
	Mar. 31, 2018
Disclosure of detailed information about borrowings [abstract]
Schedule of undrawn committed borrowing facilities	An analysis of the maturity of these undrawn committed facilities is shown below: 2018 2017 £m £m Undrawn committed borrowing facilities expiring: Less than 1 year — — In 1 to 2 years 3,910 1,115 In 2 to 3 years — 2,388 In 3 to 4 years — — In 4 to 5 years 1,773 1,773 More than 5 years — — 5,683 5,276

Sensitivites (Tables)

Sensitivites (Tables)	12 Months Ended
	Mar. 31, 2018
Disclosure of sensitivity analysis for actuarial assumptions [abstract]
Schedule of sensitivity analysis for actuarial assumptions	The currency exposure on other financial instruments is as follows: 2018 2017 Sterling £m Euro £m Dollar £m Other £m Total £m Sterling £m Euro £m Dollar £m Other £m Total £m Trade and other receivables 253 — 1,528 — 1,781 83 — 1,660 — 1,743 Trade and other payables (1,124 ) — (1,793 ) — (2,917 ) (1,209 ) — (1,795 ) — (3,004 ) Other non-current liabilities (144 ) — (254 ) — (398 ) (100 ) — (315 ) — (415 ) During 2018 and 2017, derivative financial instruments were used to manage foreign currency risk as follows: 2018 2017 Sterling £m Euro £m Dollar £m Other £m Total £m Sterling £m Euro £m Dollar £m Other £m Total £m Cash and cash equivalents 294 2 33 — 329 1,110 — 29 — 1,139 Financial investments 1,471 69 1,125 29 2,694 6,824 98 1,753 66 8,741 Borrowings (10,912 ) (3,794 ) (11,068 ) (851 ) (26,625 ) (11,099 ) (5,373 ) (10,729 ) (1,437 ) (28,638 ) Pre-derivative position (9,147 ) (3,723 ) (9,910 ) (822 ) (23,602 ) (3,165 ) (5,275 ) (8,947 ) (1,371 ) (18,758 ) Derivative effect 3,748 3,793 (7,992 ) 1,051 600 2,310 6,241 (10,708 ) 1,641 (516 ) Net debt position (5,399 ) 70 (17,902 ) 229 (23,002 ) (855 ) 966 (19,655 ) 270 (19,274 ) 2018 2017 Income statement £m Other equity reserves £m Income statement £m Other equity reserves £m Financial risk (post-tax) 1 : UK RPI change of 0.5% 2 30 — 28 — UK interest rates change of 0.5% 43 26 64 35 US interest rates change of 0.5% 39 17 61 22 US dollar exchange rate change of 10% 3 48 479 46 604 1. Excludes sensitivities to the Further Acquisition Agreement derivative. Further details on sensitivities are provided in note 30(f). 2. Excludes sensitivities to LPI curve. Further details on sensitivities are provided in note 30(f). 3. The other equity reserves impact does not reflect the exchange translation in our US subsidiaries’ net assets. It is estimated this would change by £1,040 million ( 2017 : £988 million ) in the opposite direction if the dollar exchange rate changed by 10% . Additional sensitivities in respect to commodity price risk and to our derivative fair values are as follows: 2018 2017 Income statement £m Net assets £m Income statement £m Net assets £m Commodity risk 1 (post-tax): 10% increase in commodity prices 23 23 28 28 10% decrease in commodity prices (23 ) (23 ) (29 ) (29 ) Assets and liabilities carried at fair value (pre-tax) 2 : 10% fair value change in derivative financial instruments 3 60 60 (52 ) (52 ) 10% fair value change in commodity contract derivative liabilities (5 ) (5 ) (6 ) (6 ) 1. Represents potential impact on fair values of commodity contract derivatives only. 2. Excludes sensitivities to the Further Acquisition Agreement derivative. Further details on sensitivities are provided in note 30(f). 3. The effect of a 10% change in fair value assumes no hedge accounting. These estimates are those that have a significant risk of resulting in a material adjustment to the carrying values of assets and liabilities in the next year. 2018 2017 Income statement £m Net assets £m Income statement £m Net assets £m Pensions and other post-retirement benefits 1 (pre-tax): UK discount rate change of 0.5% 2 8 1,075 9 1,305 US discount rate change of 0.5% 2 15 623 17 669 UK RPI rate change of 0.5% 3 5 965 8 1,114 UK long-term rate of increase in salaries change of 0.5% 4 — 61 2 80 US long-term rate of increase in salaries change of 0.5% 3 44 3 51 UK change of one year to life expectancy at age 65 2 588 2 673 US change of one year to life expectancy at age 65 4 359 4 365 Assumed US healthcare cost trend rates change of 1% 31 448 37 510 Environmental provision: 10% change in estimated future cash flows 154 154 175 175 0.5% change in discount rate 56 56 67 67 1. The changes shown are a change in the annual pension or other post-retirement benefit service charge and change in the defined benefit obligations. 2. A change in the discount rate is likely to occur as a result of changes in bond yields and as such would be expected to be offset to a significant degree by a change in the value of the bond assets held by the plans. 3. The projected impact resulting from a change in RPI reflects the underlying effect on pensions in payment, pensions in deferment and resultant increases in salary assumptions. 4. This change has been applied to both the pre 1 April 2014 and post 1 April 2014 rate of increase in salary assumption.

Additional disclosures in res87

Additional disclosures in respect of guaranteed securities (Tables)	12 Months Ended
	Mar. 31, 2018
Separate Financial Statements [Abstract]
Disclosure of Condensed Income Statement	Summary statements of comprehensive income for the year ended 31 March 2018 – IFRS Parent guarantor Issuer of notes Subsidiary guarantor National Grid plc £m Niagara Mohawk Power Corporation £m British Transco Finance Inc. £m National Grid Gas plc £m Other subsidiaries £m Consolidation adjustments £m National Grid consolidated £m Continuing operations Revenue — 2,416 — 1,430 11,495 (91 ) 15,250 Operating costs: Depreciation and amortisation — (190 ) — (245 ) (1,095 ) — (1,530 ) Payroll costs — (318 ) — (122 ) (1,208 ) — (1,648 ) Purchases of electricity — (537 ) — — (748 ) — (1,285 ) Purchases of gas — (166 ) — — (1,377 ) — (1,543 ) Rates and property tax — (183 ) — (94 ) (780 ) — (1,057 ) Balancing Service Incentive Scheme — — — — (1,012 ) — (1,012 ) Payments to other UK network owners — — — — (1,043 ) — (1,043 ) Other operating costs — (397 ) — (331 ) (2,002 ) 91 (2,639 ) — (1,791 ) — (792 ) (9,265 ) 91 (11,757 ) Total operating profit — 625 — 638 2,230 — 3,493 Net finance income/(costs) 889 (100 ) — (174 ) (1,360 ) — (745 ) Dividends receivable 950 — — — — (950 ) — Interest in equity accounted affiliates 1,672 — — 8 (40 ) (1,680 ) (40 ) Profit before tax 3,511 525 — 472 830 (2,630 ) 2,708 Tax 40 321 — (103 ) 626 — 884 Profit after tax from discontinued operations — — — 17 (58 ) — (41 ) Profit for the year 3,551 846 — 386 1,398 (2,630 ) 3,551 Amounts recognised in other comprehensive income from continuing operations 2 371 1 — 272 604 (877 ) 371 Total comprehensive income for the year 3,922 847 — 658 2,002 (3,507 ) 3,922 Attributable to: Equity shareholders 3,922 847 — 658 2,002 (3,507 ) 3,922 Non-controlling interests — — — — — — — 3,922 847 — 658 2,002 (3,507 ) 3,922 1. Profit for the year for British Transco Finance Inc. is £ nil as interest payable to external bond holders is offset by interest receivable on loans to National Grid Gas plc. 2. Includes other comprehensive income relating to interest in equity accounted affiliates. 34. Additional disclosures in respect of guaranteed securities continued Summary statements of comprehensive income for the year ended 31 March 2017 – IFRS Parent guarantor Issuer of notes Subsidiary guarantor National Grid plc £m Niagara Mohawk Power Corporation £m British Transco Finance Inc. £m National Grid Gas plc £m Other subsidiaries £m Consolidation adjustments £m National Grid consolidated £m Continuing operations Revenue — 2,388 — 1,376 11,435 (164 ) 15,035 Operating costs: Depreciation and amortisation — (193 ) — (256 ) (1,032 ) — (1,481 ) Payroll costs — (326 ) — (114 ) (1,138 ) — (1,578 ) Purchases of electricity — (511 ) — — (586 ) — (1,097 ) Purchases of gas — (140 ) — (67 ) (1,012 ) — (1,219 ) Rates and property tax — (188 ) — (101 ) (753 ) — (1,042 ) Balancing Service Incentive Scheme — — — — (1,120 ) — (1,120 ) Payments to other UK network owners — — — — (1,008 ) — (1,008 ) Other operating costs — (435 ) — (394 ) (2,617 ) 164 (3,282 ) — (1,793 ) — (932 ) (9,266 ) 164 (11,827 ) Total operating profit — 595 — 444 2,169 — 3,208 Net finance income/(costs) 8,177 (101 ) — (253 ) (8,910 ) — (1,087 ) Dividends receivable — — — — 8,100 (8,100 ) — Interest in equity accounted affiliates (401 ) — — — 63 401 63 Profit before tax 7,776 494 — 191 1,422 (7,699 ) 2,184 Tax 19 (181 ) — 16 (228 ) — (374 ) Profit after tax from discontinued operations — — — 4,633 1,351 — 5,984 Profit for the year 7,795 313 — 4,840 2,545 (7,699 ) 7,794 Amounts recognised in other comprehensive income from continuing operations 2 578 — — 114 177 (291 ) 578 Amounts recognised in other comprehensive income from discontinued operations 2 42 — — 51 (62 ) 11 42 Total comprehensive income for the year 8,415 313 — 5,005 2,660 (7,979 ) 8,414 Attributable to: Equity shareholders 8,415 313 — 5,005 2,661 (7,979 ) 8,415 Non-controlling interests — — — — (1 ) — (1 ) 8,415 313 — 5,005 2,660 (7,979 ) 8,414 1. Profit for the year for British Transco Finance Inc. is £ nil as interest payable to external bond holders is offset by interest receivable on loans to National Grid Gas plc. 2. Includes other comprehensive income relating to interest in equity accounted affiliates. 34. Additional disclosures in respect of guaranteed securities continued Summary statements of comprehensive income for the year ended 31 March 2016 – IFRS Parent guarantor Issuer of notes Subsidiary guarantor National Grid plc £m Niagara Mohawk Power Corporation £m British Transco Finance Inc. £m National Grid Gas plc re-presented 1 £m Other subsidiaries re-presented 1 £m Consolidation adjustments 1 £m National Grid consolidated £m Continuing operations Revenue — 2,027 — 1,244 10,069 (128 ) 13,212 Operating costs: Depreciation and amortisation — (162 ) — (255 ) (894 ) — (1,311 ) Payroll costs — (260 ) — (115 ) (962 ) — (1,337 ) Purchases of electricity — (484 ) — — (828 ) — (1,312 ) Purchases of gas — (86 ) — (75 ) (806 ) — (967 ) Rates and property tax — (155 ) — (101 ) (643 ) — (899 ) Balancing Service Incentive Scheme — — — — (907 ) — (907 ) Payments to other UK network owners — — — — (971 ) — (971 ) Other operating costs — (433 ) — (173 ) (1,805 ) 128 (2,283 ) — (1,580 ) — (719 ) (7,816 ) 128 (9,987 ) Total operating profit — 447 — 525 2,253 — 3,225 Net finance income/(costs) 701 (87 ) — (132 ) (1,437 ) — (955 ) Dividends receivable — — — — 620 (620 ) — Interest in equity accounted affiliates 1,843 — — 33 59 (1,876 ) 59 Profit before tax 2,544 360 — 426 1,495 (2,496 ) 2,329 Tax 47 (141 ) — (56 ) (277 ) — (427 ) Profit after tax from discontinued operations — — — 735 (43 ) — 692 Profit for the year 2,591 219 — 1,105 1,175 (2,496 ) 2,594 Amounts recognised in other comprehensive income from continuing operations 3 502 (1 ) — 8 426 (433 ) 502 Amounts recognised in other comprehensive income from discontinued operations 3 71 — — (13 ) 153 (140 ) 71 Total comprehensive income for the year 3,164 218 — 1,100 1,754 (3,069 ) 3,167 Attributable to: Equity shareholders 3,164 218 — 1,100 1,751 (3,069 ) 3,164 Non-controlling interests — — — — 3 — 3 3,164 218 — 1,100 1,754 (3,069 ) 3,167 1. Amounts have been re-presented to reflect the classification of the UK Gas Distribution business as a discontinued operation. 2. Profit for the year for British Transco Finance Inc. is £ nil as interest payable to external bond holders is offset by interest receivable on loans to National Grid Gas plc. 3. Includes other comprehensive income relating to interest in equity accounted affiliates.
Disclosure of Condensed Balance Sheet	Statements of financial position as at 31 March 2018 – IFRS Parent Issuer of notes Subsidiary guarantor National Grid plc £m Niagara Mohawk Power Corporation £m British Transco Finance Inc. £m National Grid Gas plc £m Other subsidiaries £m Consolidation adjustments £m National Grid consolidated £m Non-current assets Goodwill — 691 — — 4,753 — 5,444 Other intangible assets — 3 — 107 789 — 899 Property, plant and equipment — 6,148 — 4,433 29,272 — 39,853 Other non-current assets — 3 — 94 18 — 115 Amounts owed by subsidiary undertakings 350 — — 3,426 2,593 (6,369 ) — Pension assets — 231 — 412 766 — 1,409 Financial and other investments 21,708 30 — 101 12,340 (31,112 ) 3,067 Derivative financial assets 18 2 — 591 708 — 1,319 Total non-current assets 22,076 7,108 — 9,164 51,239 (37,481 ) 52,106 Current assets Inventories and current intangible assets — 36 — 22 283 — 341 Trade and other receivables 1 515 — 276 2,006 — 2,798 Current tax assets — — — — 307 (193 ) 114 Amounts owed by subsidiary undertakings 11,254 130 220 708 11,253 (23,565 ) — Financial and other investments 938 15 — 863 878 — 2,694 Derivative financial assets 308 7 — 46 — 44 405 Cash and cash equivalents — 4 — 271 54 — 329 Total current assets 12,501 707 220 2,186 14,781 (23,714 ) 6,681 Total assets 34,577 7,815 220 11,350 66,020 (61,195 ) 58,787 Current liabilities Borrowings (781 ) (51 ) (218 ) (675 ) (2,722 ) — (4,447 ) Derivative financial liabilities (187 ) (36 ) — (68 ) (66 ) (44 ) (401 ) Trade and other payables (62 ) (318 ) — (347 ) (2,726 ) — (3,453 ) Amounts owed to subsidiary undertakings (11,809 ) — — (624 ) (11,132 ) 23,565 — Current tax liabilities — (202 ) — (26 ) (88 ) 193 (123 ) Provisions — (23 ) — (66 ) (184 ) — (273 ) Total current liabilities (12,839 ) (630 ) (218 ) (1,806 ) (16,918 ) 23,714 (8,697 ) Non-current liabilities Borrowings (773 ) (2,087 ) — (3,635 ) (15,683 ) — (22,178 ) Derivative financial liabilities (41 ) (18 ) — (157 ) (444 ) — (660 ) Other non-current liabilities — (281 ) — (181 ) (855 ) — (1,317 ) Amounts owed to subsidiary undertakings (2,091 ) — — (500 ) (3,778 ) 6,369 — Deferred tax liabilities (1 ) (626 ) — (441 ) (2,568 ) — (3,636 ) Pensions and other post-retirement benefit obligations — (765 ) — — (907 ) — (1,672 ) Provisions — (248 ) — (129 ) (1,402 ) — (1,779 ) Total non-current liabilities (2,906 ) (4,025 ) — (5,043 ) (25,637 ) 6,369 (31,242 ) Total liabilities (15,745 ) (4,655 ) (218 ) (6,849 ) (42,555 ) 30,083 (39,939 ) Net assets 18,832 3,160 2 4,501 23,465 (31,112 ) 18,848 Equity Share capital 452 133 — 45 182 (360 ) 452 Share premium account 1,321 2,194 — 204 9,032 (11,430 ) 1,321 Retained earnings 21,599 830 2 2,929 14,217 (17,978 ) 21,599 Other equity reserves (4,540 ) 3 — 1,323 18 (1,344 ) (4,540 ) Shareholders’ equity 18,832 3,160 2 4,501 23,449 (31,112 ) 18,832 Non-controlling interests — — — — 16 — 16 Total equity 18,832 3,160 2 4,501 23,465 (31,112 ) 18,848 34. Additional disclosures in respect of guaranteed securities continued Statements of financial position as at 31 March 2017 – IFRS Parent Issuer of notes Subsidiary guarantor National Grid plc £m Niagara Mohawk Power Corporation 1 £m British Transco Finance Inc. £m National Grid Gas plc £m Other subsidiaries 1 £m Consolidation adjustments £m National Grid consolidated 1 £m Non-current assets Goodwill — 763 — — 5,333 — 6,096 Other intangible assets — — — 125 798 — 923 Property, plant and equipment — 6,553 — 4,358 28,914 — 39,825 Other non-current assets — 5 — 9 55 — 69 Amounts owed by subsidiary undertakings 342 — 239 3,426 2,576 (6,583 ) — Pension assets — 223 — 45 335 — 603 Financial and other investments 17,689 31 — 95 12,429 (27,061 ) 3,183 Derivative financial assets 149 2 — 813 603 — 1,567 Total non-current assets 18,180 7,577 239 8,871 51,043 (33,644 ) 52,266 Current assets Inventories and current intangible assets — 38 — 20 345 — 403 Trade and other receivables — 494 — 360 1,874 — 2,728 Current tax assets — — — — 317 — 317 Amounts owed by subsidiary undertakings 12,734 505 6 1,965 12,083 (27,293 ) — Financial and other investments 5,471 29 — 1,835 1,406 — 8,741 Derivative financial assets 202 13 — 51 174 (194 ) 246 Cash and cash equivalents 1,090 4 — (9 ) 54 — 1,139 Total current assets 19,497 1,083 6 4,222 16,253 (27,487 ) 13,574 Total assets 37,677 8,660 245 13,093 67,296 (61,131 ) 65,840 Current liabilities Borrowings (1,120 ) (55 ) (5 ) (833 ) (3,483 ) — (5,496 ) Derivative financial liabilities (533 ) (42 ) — (185 ) (581 ) 194 (1,147 ) Trade and other payables (46 ) (311 ) — (342 ) (2,646 ) — (3,345 ) Amounts owed to subsidiary undertakings (12,012 ) — — (2,151 ) (13,130 ) 27,293 — Current tax liabilities (3 ) (156 ) — (9 ) 61 — (107 ) Provisions — — — (184 ) (232 ) — (416 ) Total current liabilities (13,714 ) (564 ) (5 ) (3,704 ) (20,011 ) 27,487 (10,511 ) Non-current liabilities Borrowings (1,262 ) (2,345 ) (239 ) (3,879 ) (15,417 ) — (23,142 ) Derivative financial liabilities (272 ) (26 ) — (234 ) (714 ) — (1,246 ) Other non-current liabilities — (324 ) — (204 ) (842 ) — (1,370 ) Amounts owed to subsidiary undertakings (2,058 ) — — (756 ) (3,769 ) 6,583 — Deferred tax liabilities (3 ) (1,178 ) — (369 ) (2,929 ) — (4,479 ) Pensions and other post-retirement benefit obligations — (889 ) — — (1,647 ) — (2,536 ) Provisions — (298 ) — (104 ) (1,770 ) — (2,172 ) Total non-current liabilities (3,595 ) (5,060 ) (239 ) (5,546 ) (27,088 ) 6,583 (34,945 ) Total liabilities (17,309 ) (5,624 ) (244 ) (9,250 ) (47,099 ) 34,070 (45,456 ) Net assets 20,368 3,036 1 3,843 20,197 (27,061 ) 20,384 Equity Share capital 449 149 — 45 182 (376 ) 449 Share premium account 1,324 2,431 — 204 8,033 (10,668 ) 1,324 Retained earnings 22,582 456 1 2,268 11,914 (14,639 ) 22,582 Other equity reserves (3,987 ) — — 1,326 52 (1,378 ) (3,987 ) Shareholders’ equity 20,368 3,036 1 3,843 20,181 (27,061 ) 20,368 Non-controlling interests — — — — 16 — 16 Total equity 20,368 3,036 1 3,843 20,197 (27,061 ) 20,384 1. Consistent with the presentation of the Group balance sheet we have re-presented commodity derivatives from current and non-current receivables and payables to derivative financial assets and liabilities.
Disclosure Of Condensed Cash Flow Statement	Cash flow statements Parent Issuer of notes Subsidiary guarantor National Grid plc £m Niagara Mohawk Power Corporation £m British Transco Finance Inc. £m National Grid Gas plc £m Other subsidiaries £m Consolidation adjustments £m National Grid consolidated £m Year ended 31 March 2018 Net cash flow from operating activities – continuing operations 35 662 — 888 3,125 — 4,710 Net cash flow from operating activities – discontinued operations — — — (98 ) (109 ) — (207 ) Net cash flow from/(used in) investing activities – continuing operations 4,660 (473 ) 15 656 (1,759 ) (862 ) 2,237 Net cash flow from/(used in) investing activities – discontinued operations — — — — — — — Net cash flow (used in)/from financing activities – continuing operations (5,785 ) (189 ) (15 ) (1,041 ) (1,148 ) 862 (7,316 ) Net cash flow (used in)/from financing activities – discontinued operations — — — (125 ) (106 ) — (231 ) Net increase/(decrease) in cash and cash equivalents in the year (1,090 ) — — 280 3 — (807 ) Year ended 31 March 2017 Net cash flow from operating activities – continuing operations 53 757 — 918 2,592 — 4,320 Net cash flow from operating activities – discontinued operations — — — 450 459 — 909 Net cash flow from/(used in) investing activities – continuing operations 4,181 (469 ) 15 215 (1,118 ) (6,458 ) (3,634 ) Net cash flow from/(used in) investing activities – discontinued operations — — — 5,618 (6,298 ) — (680 ) Net cash flow (used in)/from financing activities – continuing operations (3,146 ) (288 ) (15 ) (8,322 ) 3,771 6,458 (1,542 ) Net cash flow (used in)/from financing activities – discontinued operations — — — 1,120 491 — 1,611 Net increase/(decrease) in cash and cash equivalents in the year 1,088 — — (1 ) (103 ) — 984 Year ended 31 March 2016 Net cash flow from operating activities – continuing operations 57 580 — 599 3,056 — 4,292 Net cash flow from operating activities – discontinued operations — — — 1,144 (68 ) — 1,076 Net cash flow from/(used in) investing activities – continuing operations 502 (440 ) 13 56 (1,721 ) (1,869 ) (3,459 ) Net cash flow from/(used in) investing activities – discontinued operations — — — (562 ) (15 ) — (577 ) Net cash flow (used in)/from financing activities – continuing operations (555 ) (148 ) (13 ) (1,185 ) (1,173 ) 1,869 (1,205 ) Net cash flow (used in)/from financing activities – discontinued operations — — — (63 ) (60 ) — (123 ) Net (decrease)/increase in cash and cash equivalents in the year 4 (8 ) — (11 ) 19 — 4
Schedule of borrowings	2018 2017 £m £m Current Bank loans 2,020 1,339 Bonds 2,156 2,209 Commercial paper 206 1,881 Finance leases 64 66 Other loans 1 1 4,447 5,496 Non-current Bank loans 2,384 2,343 Bonds 19,418 20,368 Finance leases 207 242 Other loans 169 189 22,178 23,142 Total borrowings 26,625 28,638 20. Borrowings continued Total borrowings are repayable as follows: 2018 2017 £m £m Less than 1 year 4,447 5,496 In 1 to 2 years 1,694 1,941 In 2 to 3 years 2,347 1,821 In 3 to 4 years 1,873 2,453 In 4 to 5 years 1,469 1,921 More than 5 years: By instalments 1,010 1,043 Other than by instalments 13,785 13,963 26,625 28,638 Maturity analysis of parent Company borrowings 2018 2017 £m £m Total borrowings are repayable as follows: Less than 1 year 781 1,120 In 1 to 2 years 438 515 In 2 to 3 years — 425 In 3 to 4 years 335 — In 4 to 5 years — 322 More than 5 years — — 1,554 2,382

Basis of preparation and rece88

Basis of preparation and recent accounting developments - Disposal of UK Gas Distribution (Details) - director	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Disclosure of subsidiaries [line items]
Number of board members appointed by entity			4
Number of directors on board			12
UK Gas Distribution (Cadent)
Disclosure of subsidiaries [line items]
Proportion of ownership interest disposed			61.00%
Proportion of ownership interest in associate		39.00%	39.00%
Quadgas HoldCo Limited
Disclosure of subsidiaries [line items]
Proportion of ownership interest in associate	39.00%	39.00%
Proportion of ownership interest in associate, put/call option equity interest		14.00%	14.00%

Basis of preparation and rece89

Basis of preparation and recent accounting developments - Impact of new IFRS accounting standards (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017		Mar. 31, 2016		Mar. 31, 2019
Disclosure of expected impact of initial application of new standards or interpretations [line items]
Revenue	£ 15,250	£ 15,035	[1]	£ 13,212	[2]
Profit (loss)	3,551	7,794	[1]	2,594	[2]
Retained earnings	21,599	22,582	[3]
Deferred tax asset	(3,636)	£ (4,479)		£ (4,634)
Scenario, Forecast | IFRS 9
Disclosure of expected impact of initial application of new standards or interpretations [line items]
Financial assets that will be reallocated to available-for-sale investments to FVTPL						£ 2,000
Increase (decrease) in financial assets on initial application of IFRS 9						(50)
Scenario, Forecast | IFRS 15
Disclosure of expected impact of initial application of new standards or interpretations [line items]
Retained earnings						167
Deferred income						240
Deferred tax asset						£ 73
Pro Forma | IFRS 15, Pass-Through
Disclosure of expected impact of initial application of new standards or interpretations [line items]
Revenue	1,056
Pro Forma | IFRS 15 - Connections
Disclosure of expected impact of initial application of new standards or interpretations [line items]
Revenue	83
Profit (loss)	£ 56
[1]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).
[2]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).
[3]	Comparative amounts have been represented to reflect the reclassification of commodity derivative contracts from trade and other receivables and payables, and from other non-current assets and liabilities to derivative financial assets and derivative financial liabilities (see note 16).

Segmental analysis - Revenue (D

Segmental analysis - Revenue (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017		Mar. 31, 2016
Disclosure of operating segments [line items]
Revenue	£ 15,250	£ 15,035	[1]	£ 13,212	[2]
UK
Disclosure of operating segments [line items]
Revenue	5,938	6,064		5,619
US
Disclosure of operating segments [line items]
Revenue	9,312	8,971		7,593
UK Electricity Transmission
Disclosure of operating segments [line items]
Revenue	4,126	4,410		3,957
UK Gas Transmission
Disclosure of operating segments [line items]
Revenue	1,082	981		938
US Regulated
Disclosure of operating segments [line items]
Revenue	9,272	8,931		7,493
NG Ventures and Other
Disclosure of operating segments [line items]
Revenue	770	713		824
Operating Segments And Unallocated Amounts
Disclosure of operating segments [line items]
Revenue	15,293	15,163		13,341
Operating segments | UK Electricity Transmission
Disclosure of operating segments [line items]
Revenue	4,154	4,439		3,977
Operating segments | UK Gas Transmission
Disclosure of operating segments [line items]
Revenue	1,091	1,080		1,047
Operating segments | US Regulated
Disclosure of operating segments [line items]
Revenue	9,272	8,931		7,493
Unallocated amounts | NG Ventures and Other
Disclosure of operating segments [line items]
Revenue	776	713		824
Unallocated amounts | NG Ventures
Disclosure of operating segments [line items]
Revenue	593	604		719
Sales between segments
Disclosure of operating segments [line items]
Revenue	(43)	(128)		(129)
Sales between segments | UK Electricity Transmission
Disclosure of operating segments [line items]
Revenue	(28)	(29)		(20)
Sales between segments | UK Gas Transmission
Disclosure of operating segments [line items]
Revenue	(9)	(99)		(109)
Sales between segments | US Regulated
Disclosure of operating segments [line items]
Revenue	0	0		0
Sales between segments | NG Ventures and Other
Disclosure of operating segments [line items]
Revenue	£ (6)	£ 0		£ 0
[1]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).
[2]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).

Segmental analysis - Operating

Segmental analysis - Operating profit from continuing operations (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017		Mar. 31, 2016
Disclosure of operating segments [line items]
Operating profit from continuing operations	£ 3,493	£ 3,208	[1]	£ 3,225	[2]
Before exceptional items and remeasurements
Disclosure of operating segments [line items]
Operating profit from continuing operations	3,457	3,773	[1]	3,214	[2]
UK
Disclosure of operating segments [line items]
Operating profit from continuing operations	1,840	1,988		2,007
UK | Before exceptional items and remeasurements
Disclosure of operating segments [line items]
Operating profit from continuing operations	1,840	2,118		2,007
US
Disclosure of operating segments [line items]
Operating profit from continuing operations	1,653	1,220		1,218
US | Before exceptional items and remeasurements
Disclosure of operating segments [line items]
Operating profit from continuing operations	1,617	1,655		1,207
Operating segments | UK Electricity Transmission
Disclosure of operating segments [line items]
Operating profit from continuing operations	1,041	1,361		1,173
Operating segments | UK Electricity Transmission | Before exceptional items and remeasurements
Disclosure of operating segments [line items]
Operating profit from continuing operations	1,041	1,372		1,173
Operating segments | UK Gas Transmission
Disclosure of operating segments [line items]
Operating profit from continuing operations	487	507		486
Operating segments | UK Gas Transmission | Before exceptional items and remeasurements
Disclosure of operating segments [line items]
Operating profit from continuing operations	487	511		486
Operating segments | US Regulated
Disclosure of operating segments [line items]
Operating profit from continuing operations	1,734	1,278		1,196
Operating segments | US Regulated | Before exceptional items and remeasurements
Disclosure of operating segments [line items]
Operating profit from continuing operations	1,698	1,713		1,185
Unallocated amounts | NG Ventures and Other
Disclosure of operating segments [line items]
Operating profit from continuing operations	231	62		370
Unallocated amounts | NG Ventures and Other | Before exceptional items and remeasurements
Disclosure of operating segments [line items]
Operating profit from continuing operations	£ 231	£ 177		£ 370
[1]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).
[2]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).

Segmental analysis - Reconcile

Segmental analysis - Reconcile total operating profit to profit before tax from continuing operations (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017		Mar. 31, 2016
Disclosure of operating segments [line items]
Operating profit from continuing operations	£ 3,493	£ 3,208	[1]	£ 3,225	[2]
Finance income	154	53	[1]	22	[2]
Finance costs	(899)	(1,140)	[1]	(977)	[2]
Share of post-tax results of joint ventures and associates	(40)	63	[1]	59	[2]
Profit before tax	2,708	2,184	[1]	2,329	[2]
NG Ventures and Other
Disclosure of operating segments [line items]
Share of post-tax results of joint ventures and associates	49	63		59
NG Ventures
Disclosure of operating segments [line items]
Operating profit from continuing operations	234	239		394
Before exceptional items and remeasurements
Disclosure of operating segments [line items]
Operating profit from continuing operations	3,457	3,773	[1]	3,214	[2]
Finance income	154	53	[1]	22	[2]
Finance costs	(1,128)	(1,082)	[1]	(878)	[2]
Share of post-tax results of joint ventures and associates	167	63	[1]	59	[2]
Profit before tax	2,650	2,807	[1]	2,417	[2]
Before exceptional items and remeasurements | NG Ventures and Other
Disclosure of operating segments [line items]
Share of post-tax results of joint ventures and associates	44	63		59
UK Gas Distribution (Cadent)
Disclosure of operating segments [line items]
Share of post-tax results of joint ventures and associates	(89)	0		0
UK Gas Distribution (Cadent) | Before exceptional items and remeasurements
Disclosure of operating segments [line items]
Share of post-tax results of joint ventures and associates	£ 123	£ 0		£ 0
[1]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).
[2]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).

Segmental analysis - Capital ex

Segmental analysis - Capital expenditure (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Disclosure of operating segments [line items]
Net book value of property, plant and equipment and other intangible assets	£ 40,752	£ 40,748	£ 35,828
Capital expenditure	4,074	3,735	3,327
Depreciation and amortisation	(1,530)	(1,481)	(1,311)
Property Plant and Equipment
Disclosure of operating segments [line items]
Net book value of property, plant and equipment and other intangible assets	39,853	39,825	35,074
Capital expenditure	3,901	3,507	3,130
Depreciation and amortisation	(1,392)	(1,348)	(1,207)
Intangible Assets
Disclosure of operating segments [line items]
Net book value of property, plant and equipment and other intangible assets	899	923	754
Capital expenditure	173	228	197
Depreciation and amortisation	(138)	(133)	(104)
UK
Disclosure of operating segments [line items]
Net book value of property, plant and equipment and other intangible assets	18,772	18,102	17,491
Capital expenditure	1,527	1,357	1,386
Depreciation and amortisation	(804)	(753)	(715)
US
Disclosure of operating segments [line items]
Net book value of property, plant and equipment and other intangible assets	21,980	22,646	18,337
Capital expenditure	2,547	2,378	1,941
Depreciation and amortisation	(726)	(728)	(596)
NG Ventures
Disclosure of operating segments [line items]
Net book value of property, plant and equipment and other intangible assets	1,454	1,432	1,482
Capital expenditure	186	98	93
Depreciation and amortisation	(143)	(143)	(144)
Operating segments | UK Electricity Transmission
Disclosure of operating segments [line items]
Net book value of property, plant and equipment and other intangible assets	13,028	12,515	11,907
Capital expenditure	999	1,027	1,084
Depreciation and amortisation	(475)	(421)	(390)
Operating segments | UK Gas Transmission
Disclosure of operating segments [line items]
Net book value of property, plant and equipment and other intangible assets	4,280	4,165	4,140
Capital expenditure	310	214	186
Depreciation and amortisation	(194)	(186)	(178)
Operating segments | US Regulated
Disclosure of operating segments [line items]
Net book value of property, plant and equipment and other intangible assets	20,953	21,638	17,490
Capital expenditure	2,424	2,247	1,856
Depreciation and amortisation	(635)	(642)	(535)
Unallocated amounts | NG Ventures and Other
Disclosure of operating segments [line items]
Net book value of property, plant and equipment and other intangible assets	2,491	2,430	2,291
Capital expenditure	341	247	201
Depreciation and amortisation	£ (226)	£ (232)	£ (208)

Segmental analysis - Narrative

Segmental analysis - Narrative (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
UK
Disclosure of operating segments [line items]
Total non-current assets other than financial instruments and pension assets	£ 20,816	£ 20,045	£ 26,261
US
Disclosure of operating segments [line items]
Total non-current assets other than financial instruments and pension assets	£ 27,663	£ 28,951	23,774
UK Gas Distribution (Cadent)
Disclosure of operating segments [line items]
Proportion of ownership interest in associate	39.00%	39.00%
Operating segments | US Regulated
Disclosure of operating segments [line items]
Operating exceptional items and remeasurements credit (debit)	£ 0	£ 11	0
Operating segments | UK Electricity Transmission
Disclosure of operating segments [line items]
Operating exceptional items and remeasurements credit (debit)	0	4	0
Operating segments | UK Gas Transmission
Disclosure of operating segments [line items]
Operating exceptional items and remeasurements credit (debit)	36	435	11
Unallocated amounts | NG Ventures and Other
Disclosure of operating segments [line items]
Operating exceptional items and remeasurements credit (debit)	£ 0	£ 115	£ 0

Segmental analysis - Commentary

Segmental analysis - Commentary on segmental adjusted operating profit results (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017		Mar. 31, 2016
Disclosure of operating segments [line items]
Increase (decrease) in revenues	£ 215
Revenue	15,250	£ 15,035	[1]	£ 13,212	[2]
Operating profit from continuing operations	3,493	3,208	[1]	3,225	[2]
Increase (decrease) in capital expenditures	(28)
Capital expenditure	4,074	3,735		3,327
Storm costs	142
UK Electricity Transmission
Disclosure of operating segments [line items]
Increase (decrease) in revenues	(285)
Revenue	4,126	4,410		3,957
UK Gas Transmission
Disclosure of operating segments [line items]
Revenue	1,082	981		938
US Regulated
Disclosure of operating segments [line items]
Increase (decrease) in revenues	341
Revenue	9,272	8,931		7,493
NG Ventures and Other
Disclosure of operating segments [line items]
Revenue	770	713		824
Before exceptional items and remeasurements
Disclosure of operating segments [line items]
Revenue	15,250	15,035	[1]	13,212	[2]
Operating profit from continuing operations	3,457	3,773	[1]	3,214	[2]
Unallocated amounts | NG Ventures and Other
Disclosure of operating segments [line items]
Increase (decrease) in revenues	63
Revenue	776	713		824
Operating profit from continuing operations	231	62		370
Increase (decrease) in capital expenditures	94
Capital expenditure	341	247		201
Unallocated amounts | Before exceptional items and remeasurements | NG Ventures and Other
Disclosure of operating segments [line items]
Increase (decrease) in profit (loss) from operating activities	54
Operating profit from continuing operations	231	177		370
Operating segments | UK Electricity Transmission
Disclosure of operating segments [line items]
Increase (decrease) in revenues	(285)
Revenue	4,154	4,439		3,977
Operating profit from continuing operations	1,041	1,361		1,173
Increase (decrease) in revenues, excluding pass-through costs	(235)
Increase (decrease) in regulated controllable costs	35
Increase (decrease) in depreciation and amortisation	54
Capital expenditure	999	1,027		1,084
Operating segments | UK Gas Transmission
Disclosure of operating segments [line items]
Increase (decrease) in revenues	11
Revenue	1,091	1,080		1,047
Operating profit from continuing operations	487	507		486
Increase (decrease) in revenues, excluding pass-through costs	(23)
Increase (decrease) in regulated controllable costs	9
Increase (decrease) in depreciation and amortisation	8
Increase (decrease) in other operating costs	(16)
Increase (decrease) in capital expenditures	96
Capital expenditure	310	214		186
Operating segments | US Regulated
Disclosure of operating segments [line items]
Increase (decrease) in revenues	341
Revenue	9,272	8,931		7,493
Operating profit from continuing operations	1,734	1,278		1,196
Increase (decrease) in depreciation and amortisation	31
Increase (decrease) in other operating costs	34
Increase (decrease) in capital expenditures	177
Increase (decrease) in capital expenditures, constant currency	311
Capital expenditure	2,424	2,247		1,856
Increase (decrease) in revenues, foreign exchange rate impact	(534)
Increase (decrease) in profit (loss) from operating activities, foreign exchange rate impact	(102)
Increase (decrease) in revenues, excluding foreign exchange rate impact	875
Increase (decrease) in revenues, excluding foreign exchange rate impact, pass through costs	597
Increase (decrease) in revenues, excluding foreign exchange rate impact, excluding pass through costs	278
Storm costs	142
Increase (decrease) in bad debt expense	(13)
Operating segments | Before exceptional items and remeasurements | UK Electricity Transmission
Disclosure of operating segments [line items]
Increase (decrease) in profit (loss) from operating activities	(331)
Operating profit from continuing operations	1,041	1,372		1,173
Operating segments | Before exceptional items and remeasurements | UK Gas Transmission
Disclosure of operating segments [line items]
Increase (decrease) in profit (loss) from operating activities	(24)
Operating profit from continuing operations	487	511		486
Operating segments | Before exceptional items and remeasurements | US Regulated
Disclosure of operating segments [line items]
Increase (decrease) in profit (loss) from operating activities	(15)
Operating profit from continuing operations	£ 1,698	£ 1,713		£ 1,185
[1]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).
[2]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).

Segmental analysis - Commenta96

Segmental analysis - Commentary on UK regulated financial performance (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017		Mar. 31, 2016
Disclosure of operating segments [line items]
Operating profit from continuing operations	£ 3,493	£ 3,208	[1]	£ 3,225	[2]
Operating segments | UK Gas Transmission
Disclosure of operating segments [line items]
Operating profit from continuing operations	487	507		486
Operating segments | UK Electricity Transmission
Disclosure of operating segments [line items]
Operating profit from continuing operations	1,041	1,361		1,173
Before exceptional items and remeasurements
Disclosure of operating segments [line items]
Operating profit from continuing operations	3,457	3,773	[1]	3,214	[2]
Before exceptional items and remeasurements | Operating segments | UK Gas Transmission
Disclosure of operating segments [line items]
Operating profit from continuing operations	487	511		486
Before exceptional items and remeasurements | Operating segments | UK Electricity Transmission
Disclosure of operating segments [line items]
Operating profit from continuing operations	£ 1,041	£ 1,372		£ 1,173
[1]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).
[2]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).

Operating costs Analysis of exp

Operating costs Analysis of expenses by nature (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017		Mar. 31, 2016
Operating Costs [Line Items]
Depreciation and amortisation	£ 1,530	£ 1,481		£ 1,311
Payroll costs	1,648	1,578		1,337
Purchases of electricity	1,285	1,097		1,312
Purchases of gas	1,543	1,219		967
Rates and property taxes	1,057	1,042		899
Balancing services incentive scheme	1,012	1,120		907
Payments to other UK network owners	1,043	1,008		971
Other	2,639	3,282		2,283
Operating expense	11,757	11,827	[1]	9,987	[2]
Inventory consumed	367	296		274
Operating leases	102	98		91
Research and development expenditure	13	14		19
Before exceptional items and remeasurements
Operating Costs [Line Items]
Depreciation and amortisation	1,530	1,481		1,311
Payroll costs	1,648	1,578		1,337
Purchases of electricity	1,299	1,143		1,304
Purchases of gas	1,539	1,241		986
Rates and property taxes	1,057	1,042		899
Balancing services incentive scheme	1,012	1,120		907
Payments to other UK network owners	1,043	1,008		971
Other	2,665	2,649		2,283
Operating expense	11,793	11,262	[1]	9,998	[2]
Exceptional items and remeasurements
Operating Costs [Line Items]
Depreciation and amortisation	0	0		0
Payroll costs	0	0		0
Purchases of electricity	(14)	(46)		8
Purchases of gas	4	(22)		(19)
Rates and property taxes	0	0		0
Balancing services incentive scheme	0	0		0
Payments to other UK network owners	0	0		0
Other	(26)	633		0
Operating expense	£ (36)	£ 565	[1]	£ (11)	[2]
[1]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).
[2]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).

Operating costs Payroll costs (

Operating costs Payroll costs (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Analysis of income and expense [abstract]
Wages and salaries	£ 1,998	£ 1,852	£ 1,553
Social security costs	157	145	120
Defined contribution scheme costs	65	58	47
Defined benefit pension costs (see note 23)	156	151	154
Share-based payments	16	32	21
Severance costs (excluding pension costs)	7	5	4
Payroll costs including capitalised costs	2,399	2,243	1,899
Less: payroll costs capitalised	(751)	(665)	(562)
Total payroll costs	1,648	1,578	1,337
US other post retirement benefit costs	£ 46	£ 53	£ 52

Operating costs Number of emplo

Operating costs Number of employees (Details)	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Disclosure of operating segments [line items]
Number of employees, year end	23,023	22,132	21,054
Average number of employees	22,705	22,043	20,842
Country of domicile
Disclosure of operating segments [line items]
Number of employees, year end	6,517	6,265	6,224
Average number of employees	6,431	6,291	6,067
Foreign countries
Disclosure of operating segments [line items]
Number of employees, year end	16,506	15,867	14,830
Average number of employees	16,274	15,752	14,775

Operating costs Key management

Operating costs Key management personnel (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Analysis of income and expense [abstract]
Short-term employee benefits	£ 8	£ 8	£ 9
Post-employment benefits	1	1	1
Share-based payments	3	6	4
Total key management compensation	£ 12	£ 15	£ 14

Operating costs Auditors remune

Operating costs Auditors remuneration (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Analysis of income and expense [abstract]
Audit of the parent Company’s individual and consolidated financial statements	£ 2.7	£ 1.5	£ 1.3
The auditing of accounts of any associate of the Company	7.5	13.7	9.2
Other services supplied	3.9	4.6	3.6
Auditor's remuneration for audit services	14.1	19.8	14.1
Tax compliance services	0.3	0.4	0.5
Tax advisory services	0	0.1	0
Other assurance services	0.7	4.6	4.3
Services relating to corporate finance transactions not covered above	0	5.9	1.6
Other non-audit services not covered above	0.9	6.3	2.5
Auditor's remuneration for tax and other services	1.9	17.3	8.9
Total auditors’ remuneration	£ 16	£ 37.1	£ 23

Exceptional items and remeas102

Exceptional items and remeasurements (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017		Mar. 31, 2016
Disclosure of associates [line items]
Operating expense	£ (11,757)	£ (11,827)	[1]	£ (9,987)	[2]
Finance costs	(899)	(1,140)	[1]	(977)	[2]
Impairment charge against investment in Quadgas HoldCo Limited	(213)	0
Share of post-tax results of joint ventures and associates	(40)	63	[1]	59	[2]
Profit before tax	2,708	2,184	[1]	2,329	[2]
Tax expense (income) on exceptional items and remeasurements	884	(374)	[1]	(427)	[2]
Profit after tax from continuing operations	3,592	1,810	[1]	1,902	[2]
Quadgas HoldCo Limited
Disclosure of associates [line items]
Impairment charge against investment in Quadgas HoldCo Limited	(213)
Share of post-tax results of joint ventures and associates	124
Tax expense (income) on exceptional items and remeasurements	(76)
Profit after tax from continuing operations	319
Exceptional items and remeasurements
Disclosure of associates [line items]
Final settlement of LIPA MSA Transition	26
Operating expense	36	(565)	[1]	11	[2]
Finance costs	229	(58)	[1]	(99)	[2]
Remeasurements – net gains on financial instruments	1	0		0
Deferred tax arising on the reduction in US corporation tax rate	5	0		0
Share of post-tax results of joint ventures and associates	(207)	0	[1]	0	[2]
Profit before tax	58	(623)	[1]	(88)	[2]
Deferred tax expense arising from reduction in UK tax rate	0	94		162
Deferred tax expense arising from reduction in US tax rate	1,510	0		0
Tax expense (income) on exceptional items and remeasurements	1,473	292	[1]	177	[2]
Profit after tax from continuing operations	1,531	(331)	[1]	89	[2]
Exceptional items and remeasurements | Quadgas HoldCo Limited
Disclosure of associates [line items]
Impairment charge against investment in Quadgas HoldCo Limited	(213)	0		0
Exceptional items and remeasurements | Commodity contract derivatives
Disclosure of associates [line items]
Remeasurements – commodity contract derivatives	10
Exceptional items
Disclosure of associates [line items]
Environmental charges	0	(526)		0
Gas holder demolition costs	0	(107)		0
Final settlement of LIPA MSA Transition	26	0		0
Operating expense	26	(633)		0
Tax expense (income) on exceptional items and remeasurements	(9)	227		0
Profit after tax from continuing operations	1,319	(312)		162
Remeasurements
Disclosure of associates [line items]
Tax expense (income) on exceptional items and remeasurements	(28)	(29)		15
Profit after tax from continuing operations	212	(19)		(73)
Remeasurements | Commodity contract derivatives
Disclosure of associates [line items]
Remeasurements – commodity contract derivatives	£ 10	£ 68		£ 11
[1]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).
[2]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).

Exceptional items and remeas103

Exceptional items and remeasurements Narrative (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Disclosure of other provisions [line items]
Remediation costs gross undiscounted		£ 481
Real discount rate	1.00%	2.00%
Other provisions	£ 2,052	£ 2,588	£ 1,719
Derivative effect	663	(580)
Exceptional items
Disclosure of other provisions [line items]
Final settlement of LIPA MSA Transition	26	0	£ 0
Exceptional items and remeasurements
Disclosure of other provisions [line items]
Final settlement of LIPA MSA Transition	26
Further Acquisition Agreement derivative
Disclosure of other provisions [line items]
Derivative effect		£ 0
Further Acquisition Agreement derivative | Exceptional items and remeasurements
Disclosure of other provisions [line items]
Gains on change in fair value of derivatives	£ 110
Quadgas HoldCo Limited
Disclosure of other provisions [line items]
Proportion of ownership interest in associate, excluding put/call option equity interest	25.00%
Investment in associates maximum amount	£ 2,100
Proportion of ownership interest in associate, put/call option equity interest	14.00%	14.00%
Quadgas HoldCo Limited | Exceptional items and remeasurements
Disclosure of other provisions [line items]
Impairment loss on financial assets	£ 213
Later than seven years and not later than ten years | Provision for decommissioning, demolition of gas holders
Disclosure of other provisions [line items]
Other provisions	£ 104	£ 107

Finance income and costs (Detai

Finance income and costs (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017		Mar. 31, 2016
Finance Income [Abstract]
Bank deposits and other financial assets	£ 81	£ 28		£ 22
Gains on disposal of available-for-sale investments	73	25		0
Finance income	154	53	[1]	22	[2]
Net interest on pensions and other post-retirement benefit obligations	(65)	(107)		(111)
Bank loans and overdrafts	(87)	(59)		(28)
Other borrowings	(1,030)	(927)		(792)
Derivatives	12	(8)		37
Unwinding of discount on provisions	(75)	(73)		(69)
Other interest	(11)	(17)		(27)
Less: interest capitalised	128	109		112
Finance costs	(899)	(1,140)	[1]	(977)	[2]
Net finance costs from continuing operations	£ (745)	£ (1,087)		£ (955)
Capitalisation rate of borrowing costs eligible for capitalisation	4.10%	3.40%		3.30%
Tax relief on capitalised interest	£ 20	£ 18		£ 19
Net foreign exchange loss on financing activities	314	264		407
Gain (loss) on change in fair value of hedging instrument used as basis for recognising hedge ineffectiveness	(90)	(27)		34
Gain (loss) on change in fair value of hedged item used as basis for recognising hedge ineffectiveness	124	60		5
Before exceptional items and remeasurements
Finance Income [Abstract]
Finance income	154	53	[1]	22	[2]
Finance costs	(1,128)	(1,082)	[1]	(878)	[2]
Exceptional items and remeasurements
Finance Income [Abstract]
Finance income	0	0	[1]	0	[2]
Finance costs	229	(58)	[1]	(99)	[2]
Net investment hedges – undesignated forward rate risk	5	60		(34)
Derivatives not designated as hedges or ineligible for hedge accounting	180	(139)		(89)
Exceptional items and remeasurements | Fair value hedges
Finance Income [Abstract]
Gain (loss) on hedge ineffectiveness recognised in profit or loss	34	33		39
Exceptional items and remeasurements | Cash flow hedges
Finance Income [Abstract]
Gain (loss) on hedge ineffectiveness recognised in profit or loss	10	£ (12)		£ (15)
Further Acquisition Agreement derivative | Exceptional items and remeasurements
Finance Income [Abstract]
Derivatives not designated as hedges or ineligible for hedge accounting	£ 110
Quadgas HoldCo Limited
Finance Income [Abstract]
Proportion of ownership interest in associate, put/call option equity interest	14.00%	14.00%
[1]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).
[2]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).

Tax - Tax (credited)_charged to

Tax - Tax (credited)/charged to the income statement - continuing operations (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017		Mar. 31, 2016
Disclosure Of Income Taxes [Line Items]
Tax expense (income), continuing operations	£ (884)	£ 374	[1]	£ 427	[2]
Before exceptional items and remeasurements
Disclosure Of Income Taxes [Line Items]
Tax expense (income), continuing operations	589	666	[1]	604	[2]
Exceptional items and remeasurements
Disclosure Of Income Taxes [Line Items]
Tax expense (income), continuing operations	(1,473)	(292)	[1]	(177)	[2]
Exceptional tax on items not included in profit before tax
Disclosure Of Income Taxes [Line Items]
Tax expense (income), continuing operations	(1,510)	(94)		(162)
Tax on other exceptional items and remeasurements
Disclosure Of Income Taxes [Line Items]
Tax expense (income), continuing operations	£ 37	£ (198)		£ (15)
[1]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).
[2]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).

Tax - Tax as a percentage of pr

Tax - Tax as a percentage of profit before tax (Details)	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Disclosure Of Income Taxes [Line Items]
Effective tax rate – continuing operations	(32.60%)	17.10%	18.30%
Before exceptional items and remeasurements
Disclosure Of Income Taxes [Line Items]
Effective tax rate – continuing operations	22.20%	23.70%	25.00%

Tax - Tax charges (Details)

Tax - Tax charges (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017		Mar. 31, 2016
Current tax:
Total current tax from continuing operations	£ 198	£ 179		£ 253
Deferred tax:
Total deferred tax from continuing operations	(1,082)	195		174
Total tax (credit)/charge from continuing operations	(884)	374	[1]	427	[2]
UK
Current tax:
UK current tax charge	205	225		239
Corporate tax adjustment in respect of prior years	(18)	(47)		(5)
Total current tax from continuing operations	187	178		234
Deferred tax:
Deferred tax	65	(9)		(80)
Deferred tax adjustment in respect of prior years	(2)	(18)		24
Total deferred tax from continuing operations	63	(27)		(56)
Total tax (credit)/charge from continuing operations	250	151
Overseas
Current tax:
UK current tax charge	15	0		38
Corporate tax adjustment in respect of prior years	(4)	1		(19)
Total current tax from continuing operations	11	1		19
Deferred tax:
Deferred tax	(1,155)	224		229
Deferred tax adjustment in respect of prior years	10	(2)		1
Total deferred tax from continuing operations	£ (1,145)	£ 222		£ 230
[1]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).
[2]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).

Tax - Tax charged_(credited) to

Tax - Tax charged/(credited) to other comprehensive income and equity (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Deferred tax:
Tax charged/(credited) to other comprehensive income and equity	£ 495	£ 318	£ 155
Continuing operations
Current tax:
Share-based payments	(3)	(4)	(1)
Available-for-sale investments	(11)	6	5
Deferred tax:
Available-for-sale investments	(18)	8	12
Cash flow hedges	(4)	20	22
Share-based payments	1	1	0
Remeasurements of gains of pension assets and post-retirement benefit obligations1	530	277	95
Tax charged/(credited) to other comprehensive income and equity	495	308	133
Total tax recognised in the statements of comprehensive income	497	311	134
Total tax relating to share based payments recognised directly in equity	(2)	(3)	(1)
Discontinued operations
Deferred tax:
Total tax recognised in the statements of comprehensive income	0	10	23
Total tax relating to share based payments recognised directly in equity	0	£ 0	£ (1)
Internal Revenue Service (IRS)
Deferred tax:
Remeasurements of gains of pension assets and post-retirement benefit obligations1	£ 281

Tax - Effective tax rate reconc

Tax - Effective tax rate reconciliation (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017		Mar. 31, 2016
Disclosure Of Income Taxes [Line Items]
Applicable tax rate	19.00%	20.00%		20.00%
Profit before tax from continuing operations	£ 2,708	£ 2,184	[1]	£ 2,329	[2]
Profit before tax from continuing operations multiplied by UK corporation tax rate of 19% (2017: 20%; 2016: 20%)	515	437		465
Effect of:
Adjustments in respect of prior years	(14)	(67)		1
Expenses not deductible for tax purposes	21	442		114
Non-taxable income	(47)	(425)		(112)
Adjustment in respect of foreign tax rates	157	104		129
Other	1	(23)		(8)
Tax expense (income), continuing operations	£ (884)	£ 374	[1]	£ 427	[2]
Effective tax rate – continuing operations	(32.60%)	17.10%		18.30%
Before exceptional items and remeasurements
Disclosure Of Income Taxes [Line Items]
Profit before tax from continuing operations	£ 2,650	£ 2,807	[1]	£ 2,417	[2]
Profit before tax from continuing operations multiplied by UK corporation tax rate of 19% (2017: 20%; 2016: 20%)	503	561		483
Effect of:
Adjustments in respect of prior years	(22)	(67)		2
Expenses not deductible for tax purposes	20	35		25
Non-taxable income	(16)	(24)		(25)
Adjustment in respect of foreign tax rates	153	180		124
Other	(42)	(19)		(5)
Tax expense (income), continuing operations	£ 589	£ 666	[1]	£ 604	[2]
Effective tax rate – continuing operations	22.20%	23.70%		25.00%
Exceptional items and remeasurements
Disclosure Of Income Taxes [Line Items]
Profit before tax from continuing operations	£ 58	£ (623)	[1]	£ (88)	[2]
Effect of:
Tax expense (income), continuing operations	(1,473)	(292)	[1]	(177)	[2]
UK
Effect of:
Deferred tax impact of change in tax rate	(7)	(94)		(162)
Tax expense (income), continuing operations	250	151
UK | Before exceptional items and remeasurements
Effect of:
Deferred tax impact of change in tax rate	(7)	0		0
US
Effect of:
Deferred tax impact of change in tax rate	(1,510)	0		0
US | Before exceptional items and remeasurements
Effect of:
Deferred tax impact of change in tax rate	£ 0	£ 0		£ 0
[1]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).
[2]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).

Tax - Tax included within the s

Tax - Tax included within the statement of financial position (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Reconciliation of changes in deferred tax liability (asset) [abstract]
Deferred tax liability (asset) beginning of period	£ 4,479	£ 4,634
Exchange adjustments and other	(268)	481
Charged/(credited) to income statement	(1,084)	121
Charged to other comprehensive income and equity	509	316
Disposal of UK Gas Distribution		(1,073)
Deferred tax liability (asset) end of period	3,636	4,479
Accelerated tax depreciation
Reconciliation of changes in deferred tax liability (asset) [abstract]
Deferred tax liability (asset) beginning of period	7,074	7,063
Exchange adjustments and other	(559)	681
Charged/(credited) to income statement	(1,641)	402
Charged to other comprehensive income and equity	0	0
Disposal of UK Gas Distribution		(1,072)
Deferred tax liability (asset) end of period	4,874	7,074
Share- based payments
Reconciliation of changes in deferred tax liability (asset) [abstract]
Deferred tax liability (asset) beginning of period	(12)	(14)
Exchange adjustments and other	0	1
Charged/(credited) to income statement	2	0
Charged to other comprehensive income and equity	1	1
Disposal of UK Gas Distribution		0
Deferred tax liability (asset) end of period	(9)	(12)
Pensions and other post- retirement benefits
Reconciliation of changes in deferred tax liability (asset) [abstract]
Deferred tax liability (asset) beginning of period	(747)	(1,038)
Exchange adjustments and other	69	(144)
Charged/(credited) to income statement	(55)	177
Charged to other comprehensive income and equity	530	264
Disposal of UK Gas Distribution		(6)
Deferred tax liability (asset) end of period	(203)	(747)
Financial instruments
Reconciliation of changes in deferred tax liability (asset) [abstract]
Deferred tax liability (asset) beginning of period	9	(53)
Exchange adjustments and other	1	(7)
Charged/(credited) to income statement	12	23
Charged to other comprehensive income and equity	(1)	46
Disposal of UK Gas Distribution		0
Deferred tax liability (asset) end of period	21	9
Other net temporary differences
Reconciliation of changes in deferred tax liability (asset) [abstract]
Deferred tax liability (asset) beginning of period	(1,845)	(1,324)
Exchange adjustments and other	221	(50)
Charged/(credited) to income statement	598	(481)
Charged to other comprehensive income and equity	(21)	5
Disposal of UK Gas Distribution		5
Deferred tax liability (asset) end of period	(1,047)	(1,845)
Net operating losses
Reconciliation of changes in deferred tax liability (asset) [abstract]
Charged/(credited) to income statement	(293)
Deferred tax liability (asset) end of period	(390)
Environmental provisions
Reconciliation of changes in deferred tax liability (asset) [abstract]
Deferred tax liability (asset) end of period	(378)
Internal Revenue Service (IRS) | Other net temporary differences
Reconciliation of changes in deferred tax liability (asset) [abstract]
Exchange adjustments and other	£ 43	£ 143

Tax - Narrative (Details)

Tax - Narrative (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Disclosure Of Income Taxes [Line Items]
Deferred tax liability (asset)	£ 3,636	£ 4,479	£ 4,634
Deferred tax assets	390	798
Charged/(credited) to income statement	1,084	£ (121)
Net operating losses
Disclosure Of Income Taxes [Line Items]
Deferred tax liability (asset)	(390)
Charged/(credited) to income statement	£ 293

Tax - Deferred tax assets not r

Tax - Deferred tax assets not recognised (Details) - GBP (£) £ in Millions	Mar. 31, 2018	Mar. 31, 2017
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]
Unused tax losses for which no deferred tax asset recognised	£ 518	£ 375
Capital losses
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]
Unused tax losses for which no deferred tax asset recognised	510	362
Non-trade deficits
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]
Unused tax losses for which no deferred tax asset recognised	4	4
Trading losses
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]
Unused tax losses for which no deferred tax asset recognised	£ 4	£ 9

Tax - Unaudited narrative (Deta

Tax - Unaudited narrative (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017		Mar. 31, 2016	[2]
Disclosure Of Income Taxes [Line Items]
Income taxes borne	£ 336	£ 644
Income taxes collected	603	887
Tax expense (income), continuing operations	(884)	374	[1]	£ 427
Unused tax losses for which no deferred tax asset recognised	518	375
UK
Disclosure Of Income Taxes [Line Items]
UK corporation tax paid	37	129
Tax expense (income), continuing operations	250	151
Capital losses
Disclosure Of Income Taxes [Line Items]
Unused tax losses for which no deferred tax asset recognised	510	362
United Kingdom Exchequer
Disclosure Of Income Taxes [Line Items]
UK corporation tax paid	£ 900	£ 1,500
[1]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).
[2]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).

Tax - Income taxes borne (Detai

Tax - Income taxes borne (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disclosure Of Income Taxes [Line Items]
Income taxes borne	£ 336	£ 644
Value Added Tax
Disclosure Of Income Taxes [Line Items]
Income taxes borne	2
Pay As You Earn And National Insurance Contribution Tax
Disclosure Of Income Taxes [Line Items]
Income taxes borne	52
Domestic Corporation Tax
Disclosure Of Income Taxes [Line Items]
Income taxes borne	37
Business Rates Tax
Disclosure Of Income Taxes [Line Items]
Income taxes borne	222
Other Taxes
Disclosure Of Income Taxes [Line Items]
Income taxes borne	£ 23

Tax - Income taxes collected (D

Tax - Income taxes collected (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disclosure Of Income Taxes [Line Items]
Income taxes collected	£ 603	£ 887
Value Added Tax
Disclosure Of Income Taxes [Line Items]
Income taxes collected	478
Pay As You Earn And National Insurance Contribution Tax
Disclosure Of Income Taxes [Line Items]
Income taxes collected	£ 125

Tax - Reconciliation on continu

Tax - Reconciliation on continuing operations of UK total tax charge to UK corporation tax paid (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017		Mar. 31, 2016
Disclosure Of Income Taxes [Line Items]
Total tax (credit)/charge from continuing operations	£ (884)	£ 374	[1]	£ 427	[2]
Adjustment for non-cash deferred tax (charge)/credit	1,082	(195)		(174)
UK corporation tax instalment payments not payable until the following year	(123)	(107)	[3]
UK
Disclosure Of Income Taxes [Line Items]
Total tax (credit)/charge from continuing operations	250	151
Adjustment for non-cash deferred tax (charge)/credit	(63)	27		56
Adjustments for current tax credit in respect of prior years	18	47		5
UK current tax charge	205	225		£ 239
UK corporation tax instalment payments not payable until the following year	(101)	(216)
UK corporation tax instalment (refunds)/payments in respect of prior years paid in current year	(67)	120
UK corporation tax paid	£ 37	£ 129
[1]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).
[2]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).
[3]	Comparative amounts have been represented to reflect the reclassification of commodity derivative contracts from trade and other receivables and payables, and from other non-current assets and liabilities to derivative financial assets and derivative financial liabilities (see note 16).

Earnings per share (EPS) - Sche

Earnings per share (EPS) - Schedule of earnings per share (Details) - GBP (£) £ / shares in Units, £ in Millions, shares in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017		Mar. 31, 2016
Earnings per share [line items]
Basic earnings from continuing operations	£ 3,591	£ 1,810		£ 1,901
Basic earnings from discontinued operations	(41)	5,985		690
Basic total earnings	£ 3,550	£ 7,795		£ 2,591
Basic earnings per share (continuing) (in GBP per share)	£ 1.038	£ 0.481	[1]	£ 0.504	[2]
Basic earnings per share (discontinuing) (in GBP per share)	(0.012)	1.590		0.183
Total basic earnings per share (in GBP per share)	£ 1.026	£ 2.071	[1]	£ 0.687	[2]
Weighted average number of shares – basic (in shares)	3,461	3,763		3,774
Diluted earnings from continuing operations	£ 3,591	£ 1,810		£ 1,901
Diluted earnings from discontinued operations	(41)	5,985		690
Diluted total earnings	£ 3,550	£ 7,795		£ 2,591
Diluted earnings per share (continuing) (in GBP per share)	£ 1.033	£ 0.479	[1]	£ 0.502	[2]
Diluted earnings per share (discontinuing) (in GBP per share)	(0.012)	1.583		0.182
Diluted earnings per share (in GBP per share)	£ 1.021	£ 2.062	[1]	£ 0.684	[2]
Weighted average number of shares – basic (in shares)	3,461	3,763		3,774
Effect of dilutive potential ordinary shares – employee share plans (in shares)	15	17		16
Weighted average number of shares – diluted (in shares)	3,476	3,780		3,790
Weighted average number of shares – diluted (in shares)	3,476	3,780		3,790
Before exceptional items and remeasurements
Earnings per share [line items]
Basic earnings from continuing operations	£ 2,060	£ 2,141		£ 1,812
Basic earnings from discontinued operations	0	607		574
Basic total earnings	£ 2,060	£ 2,748		£ 2,386
Basic earnings per share (continuing) (in GBP per share)	£ 0.595	£ 0.569		£ 0.480
Basic earnings per share (discontinuing) (in GBP per share)	0	0.161		0.152
Total basic earnings per share (in GBP per share)	£ 0.595	£ 0.730		£ 0.632
Diluted earnings from continuing operations	£ 2,060	£ 2,141		£ 1,812
Diluted earnings from discontinued operations	0	607		574
Diluted total earnings	£ 2,060	£ 2,748		£ 2,386
Diluted earnings per share (continuing) (in GBP per share)	£ 0.593	£ 0.567		£ 0.478
Diluted earnings per share (discontinuing) (in GBP per share)	0	0.160		0.151
Diluted earnings per share (in GBP per share)	£ 0.593	£ 0.727		£ 0.629
Exceptional items and remeasurements
Earnings per share [line items]
Basic earnings from discontinued operations	£ (41)	£ 5,378		£ 116
Basic total earnings	£ 1,490	£ 5,047		£ 205
Basic earnings per share (discontinuing) (in GBP per share)	£ (0.012)	£ 1.429		£ 0.031
Total basic earnings per share (in GBP per share)	£ 0.431	£ 1.341		£ 0.055
Diluted earnings from discontinued operations	£ (41)	£ 5,378		£ 116
Diluted total earnings	£ 1,490	£ 5,047		£ 205
Diluted earnings per share (discontinuing) (in GBP per share)	£ (0.012)	£ 1.423		£ 0.031
Diluted earnings per share (in GBP per share)	£ 0.428	£ 1.335		£ 0.055
Exceptional items
Earnings per share [line items]
Basic earnings from continuing operations	£ 1,319	£ (312)		£ 162
Basic earnings per share (continuing) (in GBP per share)	£ 0.381	£ (0.083)		£ 0.043
Diluted earnings from continuing operations	£ 1,319	£ (312)		£ 162
Diluted earnings per share (continuing) (in GBP per share)	£ 0.379	£ (0.083)		£ 0.043
Remeasurements
Earnings per share [line items]
Basic earnings from continuing operations	£ 212	£ (19)		£ (73)
Basic earnings per share (continuing) (in GBP per share)	£ 0.062	£ (0.005)		£ (0.019)
Diluted earnings from continuing operations	£ 212	£ (19)		£ (73)
Diluted earnings per share (continuing) (in GBP per share)	£ 0.061	£ (0.005)		£ (0.019)
[1]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).
[2]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).

Earnings per share (EPS) - Narr

Earnings per share (EPS) - Narrative (Details) £ in Millions	Jun. 02, 2018	Jun. 02, 2017GBP (£)	Mar. 31, 2018GBP (£)	Mar. 31, 2017GBP (£)	Mar. 31, 2016GBP (£)
Summary Of Dividends [Line Items]
Dividends paid			£ 4,487	£ 1,463	£ 1,337
Stock consolidation ratio	0.9167
Special dividend
Summary Of Dividends [Line Items]
Dividends paid		£ 3,170	£ 3,171	£ 0	£ 0

Dividends (Details)

Dividends (Details) - GBP (£) £ / shares in Units, £ in Millions	Jun. 02, 2017	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Summary Of Dividends [Line Items]
Cash dividends paid (GBP per share)		£ 128.965	£ 43.51	£ 43.16
Dividends paid		£ 4,487	£ 1,463	£ 1,337
Scrip dividend		£ 209	£ 183	£ 279
Interim dividend in respect of the current year
Summary Of Dividends [Line Items]
Cash dividends paid (GBP per share)		£ 15.49	£ 15.17	£ 15
Dividends paid		£ 346	£ 540	£ 532
Scrip dividend		£ 176	£ 32	£ 31
Special dividend
Summary Of Dividends [Line Items]
Cash dividends paid (GBP per share)	£ 0.84375	£ 84.375	£ 0	£ 0
Dividends paid	£ 3,170	£ 3,171	£ 0	£ 0
Scrip dividend		£ 0	£ 0	£ 0
Final dividend in respect of the prior year
Summary Of Dividends [Line Items]
Cash dividends paid (GBP per share)		£ 29.10	£ 28.34	£ 28.16
Dividends paid		£ 970	£ 923	£ 805
Scrip dividend		£ 33	£ 151	£ 248

Dividends Narrative (Details)

Dividends Narrative (Details) £ / shares in Units, £ in Millions	Jun. 02, 2017GBP (£)£ / shares	Jun. 02, 2017$ / shares	Mar. 31, 2018GBP (£)£ / shares	Mar. 31, 2017GBP (£)£ / shares	Mar. 31, 2016GBP (£)£ / shares
Summary Of Dividends [Line Items]
Dividends declared before financial statements authorised for issue (GBP per share) | £ / shares			£ 0.3044
Dividends declared before financial statements authorised for issue | £			£ 1,000
Cash dividends paid (GBP per share) | £ / shares			£ 128.965	£ 43.51	£ 43.16
Cash dividends paid, special dividend | £			£ 4,487	£ 1,463	£ 1,337
Special dividend
Summary Of Dividends [Line Items]
Cash dividends paid (GBP per share) | £ / shares	£ 0.84375		£ 84.375	£ 0	£ 0
Cash dividends paid, special dividend | £	£ 3,170		£ 3,171	£ 0	£ 0
Special dividend - American Depositary Share
Summary Of Dividends [Line Items]
Cash dividends paid (GBP per share) | $ / shares		$ 5.4224

Discontinued operations Narrati

Discontinued operations Narrative (Details) - GBP (£) £ in Millions	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017		Mar. 31, 2016
Disclosure of subsidiaries [line items]
Liabilities in subsidiary or businesses acquired or disposed			£ 1,775
Assets other than cash or cash equivalents in subsidiary or businesses acquired or disposed			429
Gain on disposal of UK Gas Distribution after tax			5,321		£ 0
Loss after tax from discontinued operations		£ 41	(5,984)	[1]	(692)	[2]
Loss relating to completion accounts		33
Post year end movements in provisions and accruals		8
Net cash (used in)/inflows from operating activities – discontinued operations		207	(909)		(1,076)
Cash flows from (used in) financing activities, discontinued operations		231	(1,611)		123
Proceeds received from loans		1,941	2,463		2,726
Repayment of loans		£ (2,156)	(1,616)		£ (896)
Sales price consideration, discontinued operations			7,494
Transaction costs, discontinued operations			1,837
Tax credit, discontinued operations			312
Net assets on disposal, discontinued operations	£ 648		£ 648
UK Gas Distribution (Cadent)
Disclosure of subsidiaries [line items]
Proportion of ownership interest disposed			61.00%
Proportion of ownership interest in associate		39.00%	39.00%
Xoserve Limited
Disclosure of subsidiaries [line items]
Proportion of ownership interest disposed			45.00%
Quadgas HoldCo Limited
Disclosure of subsidiaries [line items]
Proportion of ownership interest in associate	39.00%	39.00%
Proportion of ownership interest in associate, excluding put/call option equity interest		25.00%
UK Gas Distribution
Disclosure of subsidiaries [line items]
Proportion of ownership interest disposed			100.00%
Cash consideration			£ 3,679
Depreciation and amortisation expense, per month			25
Discontinued operations
Disclosure of subsidiaries [line items]
Proceeds received from loans			4,800
Repayment of loans			£ 3,200
[1]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).
[2]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).

Discontinued operations (Detail

Discontinued operations (Details) - GBP (£) £ in Millions		12 Months Ended
		Mar. 31, 2018	Mar. 31, 2017		Mar. 31, 2016
Summary income statement and comprehensive income, discontinued operations [line items]
Revenue			£ 1,887		£ 1,903
Operating costs			(993)		(1,043)
Operating Profit, Discontinued Operations			894		860
Finance costs			(152)		(157)
Profit before tax from discontinued operations			742		703
Tax from discontinued operations			(79)		(11)
Profit after tax from discontinued operations			663		692
Gain on disposal of UK Gas Distribution			5,009		0
Tax on gain on disposal of UK Gas Distribution			312		0
Gain on disposal of UK Gas Distribution after tax			5,321		0
Total profit after tax from discontinued operations		£ (41)	5,984	[1]	692	[2]
Remeasurement gains of pension assets and post-retirement benefit obligations		1,313	423		410
Tax on items that will never be reclassified to profit or loss		(530)	(277)		(95)
Total items from continuing operations that will never be reclassified to profit or loss		925	146		315
Net gains in respect of cash flow hedges		19	70		88
Transferred to profit or loss in respect of cash flow hedges		(3)	(6)		26
Tax on items that may be reclassified subsequently to profit or loss	[3]	33	(34)		(39)
Total items from discontinued operations that may be reclassified subsequently to profit or loss		(554)	432		187
Other comprehensive income for the year, net of tax		371	620		573
Total comprehensive income for the year		£ 3,922	8,414		3,167
Discontinued operations
Summary income statement and comprehensive income, discontinued operations [line items]
Remeasurement gains of pension assets and post-retirement benefit obligations			(75)		129
Tax on items that will never be reclassified to profit or loss			13		(30)
Total items from continuing operations that will never be reclassified to profit or loss			(62)		99
Net gains in respect of cash flow hedges			(106)		(38)
Transferred to profit or loss in respect of cash flow hedges			233		3
Tax on items that may be reclassified subsequently to profit or loss			(23)		7
Total items from discontinued operations that may be reclassified subsequently to profit or loss			104		(28)
Other comprehensive income for the year, net of tax			42		71
Total comprehensive income for the year			£ 6,026		763
UK Gas Distribution
Summary income statement and comprehensive income, discontinued operations [line items]
Sale preparation costs					£ 22
[1]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).
[2]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).
[3]	The share of other comprehensive income of associates relates to items of other comprehensive income of Cadent (investment through Quadgas HoldCo Limited), comprising £142 million (2017: £nil; 2016: £nil) remeasurement gains on pension assets and post-retirement benefit obligations and a £5 million (2017: £nil; 2016: £nil) net gain in respect of cash flow hedges. Both items are shown net of tax.

Goodwill - Disclosure of goodwi

Goodwill - Disclosure of goodwill (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disclosure of reconciliation of changes in intangible assets and goodwill [line items]
Exchange adjustments	£ 707
Goodwill, ending balance	6,343
Goodwill
Disclosure of reconciliation of changes in intangible assets and goodwill [line items]
Goodwill, beginning balance	6,096	£ 5,315
Exchange adjustments	(652)	781
Goodwill, ending balance	£ 5,444	£ 6,096

Goodwill - Narrative (Details)

Goodwill - Narrative (Details) - GBP (£) £ in Millions	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Disclosure of reconciliation of changes in goodwill [line items]
Goodwill	£ 6,343
Growth rate used to extrapolate cash flow projections	2.30%	2.00%
Discount rate applied to cash flow projections	5.30%	5.40%
Goodwill
Disclosure of reconciliation of changes in goodwill [line items]
Goodwill	£ 5,444	£ 6,096	£ 5,315
Goodwill | Gross carrying amount
Disclosure of reconciliation of changes in goodwill [line items]
Goodwill	5,458	6,112
Goodwill | Accumulated impairment
Disclosure of reconciliation of changes in goodwill [line items]
Goodwill	14	16
Goodwill | New York | Gross carrying amount
Disclosure of reconciliation of changes in goodwill [line items]
Goodwill	3,137	3,512
Goodwill | Massachusetts | Gross carrying amount
Disclosure of reconciliation of changes in goodwill [line items]
Goodwill	1,173	1,313
Goodwill | Rhode Island | Gross carrying amount
Disclosure of reconciliation of changes in goodwill [line items]
Goodwill	436	488
Goodwill | Federal | Gross carrying amount
Disclosure of reconciliation of changes in goodwill [line items]
Goodwill	£ 699	£ 783

Other intangible assets (Detail

Other intangible assets (Details) - GBP (£) £ in Millions		12 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Changes in intangible assets other than goodwill [abstract]
Intangible assets other than goodwill, beginning balance	[1]	£ 923
Intangible assets other than goodwill, ending balance		899	£ 923	[1]
Software
Changes in intangible assets other than goodwill [abstract]
Intangible assets other than goodwill, beginning balance		923
Additions		173
Amortisation charge for the year		(138)
Intangible assets other than goodwill, ending balance		£ 899	923
US ERP System
Disclosure of detailed information about intangible assets [line items]
Useful lives or amortisation rates, Intangible assets other than goodwill, period		6 years
Changes in intangible assets other than goodwill [abstract]
Intangible assets other than goodwill, ending balance		£ 160
Gross carrying amount | Software
Changes in intangible assets other than goodwill [abstract]
Intangible assets other than goodwill, beginning balance		1,732	1,744
Exchange adjustments		(98)	105
Additions		173	234
Disposals		(18)	(43)
Disposal of UK Gas Distribution			(304)
Reclassifications		8	(4)
Intangible assets other than goodwill, ending balance		1,797	1,732
Accumulated depreciation/amortisation | Software
Changes in intangible assets other than goodwill [abstract]
Intangible assets other than goodwill, beginning balance		(809)	(857)
Exchange adjustments		43	(43)
Disposals		6	40
Disposal of UK Gas Distribution			215
Amortisation charge for the year		(138)	(164)
Intangible assets other than goodwill, ending balance		£ (898)	£ (809)
Bottom of range
Disclosure of detailed information about intangible assets [line items]
Useful lives or amortisation rates, Intangible assets other than goodwill, period		3 years
Top of range
Disclosure of detailed information about intangible assets [line items]
Useful lives or amortisation rates, Intangible assets other than goodwill, period		10 years
[1]	Comparative amounts have been represented to reflect the reclassification of commodity derivative contracts from trade and other receivables and payables, and from other non-current assets and liabilities to derivative financial assets and derivative financial liabilities (see note 16).

Property, plant and equipment -

Property, plant and equipment - Disclosure of property plant and equipment (Details) - GBP (£)		12 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Changes in property, plant and equipment [abstract]
Property, plant and equipment, beginning balance	[1]	£ 39,825,000,000
Exchange adjustments		2,428,000,000
Additions		3,901,000,000
Disposals		(45,000,000)
Depreciation charge for the year		(1,392,000,000)
Property, plant and equipment, ending balance		39,853,000,000	£ 39,825,000,000	[1]
Cost
Changes in property, plant and equipment [abstract]
Property, plant and equipment, beginning balance		56,995,000,000	62,575,000,000
Exchange adjustments		(3,187,000,000)	3,522,000,000
Additions		3,901,000,000	4,089,000,000
Disposals		(287,000,000)	(868,000,000)
Disposal of UK Gas Distribution			(12,361,000,000)
Increase (decrease) through transfers, property, plant and equipment		12,000,000	38,000,000
Property, plant and equipment, ending balance		57,434,000,000	56,995,000,000
Property, plant and equipment, assets retired from active use and not classified as held for sale		51,000,000	107,000,000
Accumulated depreciation/amortisation
Changes in property, plant and equipment [abstract]
Property, plant and equipment, beginning balance		(17,170,000,000)	(19,211,000,000)
Exchange adjustments		759,000,000	(853,000,000)
Disposals		242,000,000	790,000,000
Disposal of UK Gas Distribution			3,661,000,000
Depreciation charge for the year		(1,392,000,000)	(1,535,000,000)
Increase (decrease) through transfers, property, plant and equipment		(20,000,000)	(22,000,000)
Property, plant and equipment, ending balance		(17,581,000,000)	(17,170,000,000)
Property, plant and equipment, assets retired from active use and not classified as held for sale		51,000,000	107,000,000
Land and buildings
Changes in property, plant and equipment [abstract]
Property, plant and equipment, beginning balance		2,295,000,000
Property, plant and equipment, ending balance		2,256,000,000	2,295,000,000
Land and buildings | Cost
Changes in property, plant and equipment [abstract]
Property, plant and equipment, beginning balance		2,979,000,000	2,758,000,000
Exchange adjustments		(169,000,000)	196,000,000
Additions		38,000,000	55,000,000
Disposals		(16,000,000)	(22,000,000)
Disposal of UK Gas Distribution			(112,000,000)
Increase (decrease) through transfers, property, plant and equipment		98,000,000	104,000,000
Property, plant and equipment, ending balance		2,930,000,000	2,979,000,000
Land and buildings | Accumulated depreciation/amortisation
Changes in property, plant and equipment [abstract]
Property, plant and equipment, beginning balance		(684,000,000)	(640,000,000)
Exchange adjustments		28,000,000	(29,000,000)
Disposals		10,000,000	42,000,000
Disposal of UK Gas Distribution			29,000,000
Depreciation charge for the year		(28,000,000)	(84,000,000)
Increase (decrease) through transfers, property, plant and equipment		0	(2,000,000)
Property, plant and equipment, ending balance		(674,000,000)	(684,000,000)
Plant and machinery
Changes in property, plant and equipment [abstract]
Property, plant and equipment, beginning balance		33,235,000,000
Property, plant and equipment, ending balance		32,976,000,000	33,235,000,000
Plant and machinery | Cost
Changes in property, plant and equipment [abstract]
Property, plant and equipment, beginning balance		49,231,000,000	54,772,000,000
Exchange adjustments		(2,862,000,000)	3,157,000,000
Additions		430,000,000	822,000,000
Disposals		(216,000,000)	(572,000,000)
Disposal of UK Gas Distribution			(11,861,000,000)
Increase (decrease) through transfers, property, plant and equipment		2,791,000,000	2,913,000,000
Property, plant and equipment, ending balance		49,374,000,000	49,231,000,000
Plant and machinery | Accumulated depreciation/amortisation
Changes in property, plant and equipment [abstract]
Property, plant and equipment, beginning balance		(15,996,000,000)	(17,828,000,000)
Exchange adjustments		695,000,000	(780,000,000)
Disposals		199,000,000	545,000,000
Disposal of UK Gas Distribution			3,425,000,000
Depreciation charge for the year		(1,276,000,000)	(1,338,000,000)
Increase (decrease) through transfers, property, plant and equipment		(20,000,000)	(20,000,000)
Property, plant and equipment, ending balance		(16,398,000,000)	(15,996,000,000)
Assets in the course of construction
Changes in property, plant and equipment [abstract]
Property, plant and equipment, beginning balance		3,951,000,000
Property, plant and equipment, ending balance		4,273,000,000	3,951,000,000
Assets in the course of construction | Cost
Changes in property, plant and equipment [abstract]
Property, plant and equipment, beginning balance		3,951,000,000	3,874,000,000
Exchange adjustments		(89,000,000)	93,000,000
Additions		3,358,000,000	3,080,000,000
Disposals		(21,000,000)	(70,000,000)
Disposal of UK Gas Distribution			(88,000,000)
Increase (decrease) through transfers, property, plant and equipment		(2,926,000,000)	(2,938,000,000)
Property, plant and equipment, ending balance		4,273,000,000	3,951,000,000
Assets in the course of construction | Accumulated depreciation/amortisation
Changes in property, plant and equipment [abstract]
Property, plant and equipment, beginning balance		0	0
Exchange adjustments		0	0
Disposals		0	0
Disposal of UK Gas Distribution			0
Depreciation charge for the year		0	0
Increase (decrease) through transfers, property, plant and equipment		0	0
Property, plant and equipment, ending balance		0	0
Motor vehicles and office equipment
Changes in property, plant and equipment [abstract]
Property, plant and equipment, beginning balance		344,000,000
Property, plant and equipment, ending balance		348,000,000	344,000,000
Motor vehicles and office equipment | Cost
Changes in property, plant and equipment [abstract]
Property, plant and equipment, beginning balance		834,000,000	1,171,000,000
Exchange adjustments		(67,000,000)	76,000,000
Additions		75,000,000	132,000,000
Disposals		(34,000,000)	(204,000,000)
Disposal of UK Gas Distribution			(300,000,000)
Increase (decrease) through transfers, property, plant and equipment		49,000,000	(41,000,000)
Property, plant and equipment, ending balance		857,000,000	834,000,000
Motor vehicles and office equipment | Accumulated depreciation/amortisation
Changes in property, plant and equipment [abstract]
Property, plant and equipment, beginning balance		(490,000,000)	(743,000,000)
Exchange adjustments		36,000,000	(44,000,000)
Disposals		33,000,000	203,000,000
Disposal of UK Gas Distribution			207,000,000
Depreciation charge for the year		(88,000,000)	(113,000,000)
Increase (decrease) through transfers, property, plant and equipment		0	0
Property, plant and equipment, ending balance		(509,000,000)	£ (490,000,000)
KeySpan
Changes in property, plant and equipment [abstract]
Disposals		0
KeySpan | Cost
Changes in property, plant and equipment [abstract]
Disposals		(334,000,000)
KeySpan | Accumulated depreciation/amortisation
Changes in property, plant and equipment [abstract]
Disposals		£ (334,000,000)
[1]	Comparative amounts have been represented to reflect the reclassification of commodity derivative contracts from trade and other receivables and payables, and from other non-current assets and liabilities to derivative financial assets and derivative financial liabilities (see note 16).

Property, plant and equipmen127

Property, plant and equipment - Other Disclosures (Details) - GBP (£)	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Disclosure of detailed information about property, plant and equipment [line items]
Interest costs capitalised	£ 128,000,000	£ 109,000,000	£ 112,000,000
Increase (decrease) through transfers, property, plant and equipment	45,000,000
Cost
Disclosure of detailed information about property, plant and equipment [line items]
Increase (decrease) through transfers, property, plant and equipment	287,000,000	868,000,000
Accumulated depreciation and amortisation [member]
Disclosure of detailed information about property, plant and equipment [line items]
Increase (decrease) through transfers, property, plant and equipment	(242,000,000)	(790,000,000)
Land and buildings | Cost
Disclosure of detailed information about property, plant and equipment [line items]
Increase (decrease) through transfers, property, plant and equipment	16,000,000	22,000,000
Land and buildings | Accumulated depreciation and amortisation [member]
Disclosure of detailed information about property, plant and equipment [line items]
Increase (decrease) through transfers, property, plant and equipment	(10,000,000)	(42,000,000)
Motor vehicles and office equipment
Disclosure of detailed information about property, plant and equipment [line items]
Recognised finance lease as assets	253,000,000	289,000,000
Additions to recognised finance lease as assets	58,000,000	98,000,000
Motor vehicles and office equipment | Cost
Disclosure of detailed information about property, plant and equipment [line items]
Increase (decrease) through transfers, property, plant and equipment	34,000,000	204,000,000
Motor vehicles and office equipment | Accumulated depreciation and amortisation [member]
Disclosure of detailed information about property, plant and equipment [line items]
Increase (decrease) through transfers, property, plant and equipment	£ (33,000,000)	(203,000,000)
Bottom of range | Land and buildings
Disclosure of detailed information about property, plant and equipment [line items]
Useful lives or depreciation rates, property, plant and equipment, period	0 years
Bottom of range | Motor vehicles and office equipment
Disclosure of detailed information about property, plant and equipment [line items]
Useful lives or depreciation rates, property, plant and equipment, period	0 years
Bottom of range | Electricity transmission plant and wires
Disclosure of detailed information about property, plant and equipment [line items]
Useful lives or depreciation rates, property, plant and equipment, period	15 years
Bottom of range | Electricity distribution plant
Disclosure of detailed information about property, plant and equipment [line items]
Useful lives or depreciation rates, property, plant and equipment, period	29 years
Bottom of range | Electricity generation plant
Disclosure of detailed information about property, plant and equipment [line items]
Useful lives or depreciation rates, property, plant and equipment, period	20 years
Bottom of range | Interconnector plant and other
Disclosure of detailed information about property, plant and equipment [line items]
Useful lives or depreciation rates, property, plant and equipment, period	5 years
Bottom of range | Gas plant – mains, services and regulating equipment
Disclosure of detailed information about property, plant and equipment [line items]
Useful lives or depreciation rates, property, plant and equipment, period	10 years
Bottom of range | Gas plant – storage
Disclosure of detailed information about property, plant and equipment [line items]
Useful lives or depreciation rates, property, plant and equipment, period	5 years
Bottom of range | Gas plant – meters
Disclosure of detailed information about property, plant and equipment [line items]
Useful lives or depreciation rates, property, plant and equipment, period	7 years
Top of range | Land and buildings
Disclosure of detailed information about property, plant and equipment [line items]
Useful lives or depreciation rates, property, plant and equipment, period	101 years
Top of range | Motor vehicles and office equipment
Disclosure of detailed information about property, plant and equipment [line items]
Useful lives or depreciation rates, property, plant and equipment, period	29 years
Top of range | Electricity transmission plant and wires
Disclosure of detailed information about property, plant and equipment [line items]
Useful lives or depreciation rates, property, plant and equipment, period	100 years
Top of range | Electricity distribution plant
Disclosure of detailed information about property, plant and equipment [line items]
Useful lives or depreciation rates, property, plant and equipment, period	75 years
Top of range | Electricity generation plant
Disclosure of detailed information about property, plant and equipment [line items]
Useful lives or depreciation rates, property, plant and equipment, period	93 years
Top of range | Interconnector plant and other
Disclosure of detailed information about property, plant and equipment [line items]
Useful lives or depreciation rates, property, plant and equipment, period	60 years
Top of range | Gas plant – mains, services and regulating equipment
Disclosure of detailed information about property, plant and equipment [line items]
Useful lives or depreciation rates, property, plant and equipment, period	95 years
Top of range | Gas plant – storage
Disclosure of detailed information about property, plant and equipment [line items]
Useful lives or depreciation rates, property, plant and equipment, period	65 years
Top of range | Gas plant – meters
Disclosure of detailed information about property, plant and equipment [line items]
Useful lives or depreciation rates, property, plant and equipment, period	65 years
Property Plant and Equipment | Cost
Disclosure of detailed information about property, plant and equipment [line items]
Interest costs capitalised	£ 1,861,000,000	1,749,000,000
Trade and other payables
Disclosure of detailed information about property, plant and equipment [line items]
Contributions to cost of property, plant and equipment	85,000,000	89,000,000
Non-current liabilities
Disclosure of detailed information about property, plant and equipment [line items]
Contributions to cost of property, plant and equipment	844,000,000	£ 839,000,000
KeySpan
Disclosure of detailed information about property, plant and equipment [line items]
Increase (decrease) through transfers, property, plant and equipment	0
KeySpan | Cost
Disclosure of detailed information about property, plant and equipment [line items]
Increase (decrease) through transfers, property, plant and equipment	334,000,000
KeySpan | Accumulated depreciation and amortisation [member]
Disclosure of detailed information about property, plant and equipment [line items]
Increase (decrease) through transfers, property, plant and equipment	£ 334,000,000

Other non-current assets (Detai

Other non-current assets (Details) - GBP (£) £ in Millions	Mar. 31, 2018	Mar. 31, 2017
Subclassifications of assets, liabilities and equities [abstract]
Other receivables	£ 36	£ 45
Non-current tax assets	51	0
Prepayments and accrued income	28	24
Other non-current assets	£ 115	£ 69	[1]
[1]	Comparative amounts have been represented to reflect the reclassification of commodity derivative contracts from trade and other receivables and payables, and from other non-current assets and liabilities to derivative financial assets and derivative financial liabilities (see note 16).

Financial and other investme129

Financial and other investments (Details) - GBP (£) £ in Millions	Mar. 31, 2018	Mar. 31, 2017		Mar. 31, 2016
Disclosure of associates [line items]
Available-for-sale investments, non-current	£ 417	£ 605
Loans and receivables, non-current	482	495
Other non-current financial assets	899	1,100	[1]
Available-for-sale investments, current	2,304	7,432
Loans and receivables, current	390	1,309
Other current financial assets	2,694	8,741	[1]	£ 2,998
Other financial assets	3,593	9,841
Short term investments, restricted	69	14
Managed investments in equity and bonds, restricted	301	434
Managed investments in equity and bonds, US non-qualified plan	214	225
Quadgas HoldCo Limited
Disclosure of associates [line items]
Receivables due from associates	352	434
Investments in short-term money funds
Disclosure of associates [line items]
Other financial assets	1,999	6,899
Managed investments in equity and bonds
Disclosure of associates [line items]
Other financial assets	530	939
Cash surrender value of life insurance policies
Disclosure of associates [line items]
Other financial assets	198	202
Loans to joint ventures and associates
Disclosure of associates [line items]
Other financial assets	482	495
Restricted balances - collateral
Disclosure of associates [line items]
Other financial assets	335	1,262
Restricted balances - other
Disclosure of associates [line items]
Other financial assets	£ 49	£ 44
[1]	Comparative amounts have been represented to reflect the reclassification of commodity derivative contracts from trade and other receivables and payables, and from other non-current assets and liabilities to derivative financial assets and derivative financial liabilities (see note 16).

Financial and other investme130

Financial and other investments Unaudited commentary (Details) £ in Millions	Mar. 31, 2018GBP (£)
Disclosure of financial assets [abstract]
Decrease in current available-for-sale investment	£ 5,128
Proceeds received from sale of UK gas distribution	£ 4,010

Investments in joint venture131

Investments in joint ventures and associates Share of net assets, Joint Ventures and Associates (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018		Mar. 31, 2017		Mar. 31, 2016
Disclosure of joint ventures [line items]
Investments accounted for using equity method, beginning balance	£ 2,083	[1]	£ 397
Exchange adjustments	(12)		35
Additions	129		137
Acquisition of stake in Quadgas HoldCo Limited	0		1,611
Capitalisation of shareholder loan to Quadgas HoldCo Limited	69		0
Impairment charge against investment in Quadgas HoldCo Limited	(213)		0
Share of post-tax results for the year	173		63
Share of other comprehensive income of associates, net of tax	147		0
Dividends received	(213)		(99)		£ (72)
Other movements	5		(61)
Investments accounted for using equity method, ending balance	2,168		2,083	[1]	397
Joint ventures
Disclosure of joint ventures [line items]
Investments accounted for using equity method, beginning balance	307		313
Exchange adjustments	7		19
Additions	64		63
Acquisition of stake in Quadgas HoldCo Limited	0		0
Capitalisation of shareholder loan to Quadgas HoldCo Limited	0		0
Impairment charge against investment in Quadgas HoldCo Limited	0		0
Share of post-tax results for the year	26		48
Share of other comprehensive income of associates, net of tax	0		0
Dividends received	(43)		(75)
Other movements	0		(61)
Investments accounted for using equity method, ending balance	361		307		313
Associates
Disclosure of joint ventures [line items]
Investments accounted for using equity method, beginning balance	1,776		84
Exchange adjustments	(19)		16
Additions	65		74
Acquisition of stake in Quadgas HoldCo Limited	0		1,611
Capitalisation of shareholder loan to Quadgas HoldCo Limited	69		0
Impairment charge against investment in Quadgas HoldCo Limited	(213)		0
Share of post-tax results for the year	147		15
Share of other comprehensive income of associates, net of tax	147		0
Dividends received	(170)		(24)
Other movements	5		0
Investments accounted for using equity method, ending balance	£ 1,807		£ 1,776		£ 84
[1]	Comparative amounts have been represented to reflect the reclassification of commodity derivative contracts from trade and other receivables and payables, and from other non-current assets and liabilities to derivative financial assets and derivative financial liabilities (see note 16).

Investments in joint venture132

Investments in joint ventures and associates BritNed Development Limited, Joint venture (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017		Mar. 31, 2016
Disclosure of joint ventures [line items]
Non-current assets	£ 52,106	£ 52,266	[1]
Cash and cash equivalents	329	1,139	[1]	£ 127
Non-current liabilities	(31,242)	(34,945)	[1]
Current liabilities	(8,697)	(10,511)	[1]
Net assets	18,848	20,384	[1]
Revenue	15,250	15,035	[2]	13,212	[3]
Depreciation and amortisation	(1,530)	(1,481)		(1,311)
Other costs	(2,639)	(3,282)		(2,283)
Operating profit	3,493	3,208	[2]	3,225	[3]
Tax	884	(374)	[2]	(427)	[3]
Profit after tax from continuing operations	3,592	1,810	[2]	1,902	[3]
Group’s share of profit (National Grid ownership 50%)	(40)	63	[2]	£ 59	[3]
BritNed Development Limited
Disclosure of joint ventures [line items]
Non-current assets	390	392
Cash and cash equivalents	50	45
All other current assets	4	1
Non-current liabilities	(10)	(10)
Current liabilities	(28)	(20)
Net assets	406	408
Carrying amount of the Group’s investment (National Grid ownership 50%)	203	204
Revenue	429	399
Depreciation and amortisation	(13)	(13)
Other costs	(324)	(257)
Operating profit	92	129
Tax	(20)	(23)
Profit after tax from continuing operations	72	106
Group’s share of profit (National Grid ownership 50%)	£ 36	£ 53
[1]	Comparative amounts have been represented to reflect the reclassification of commodity derivative contracts from trade and other receivables and payables, and from other non-current assets and liabilities to derivative financial assets and derivative financial liabilities (see note 16).
[2]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).
[3]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).

Investments in joint venture133

Investments in joint ventures and associates Quadgas Holdco Limited, Associate (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017		Mar. 31, 2016
Disclosure of associates [line items]
Non-current assets	£ 52,106	£ 52,266	[1]
Current assets	6,681	13,574	[1]
Non-current liabilities	(31,242)	(34,945)	[1]
Current liabilities	(8,697)	(10,511)	[1]
Net assets	18,848	20,384	[1]
Discount for non-controlling interest	(16)	(16)	[1]
Revenue	15,250	15,035	[2]	£ 13,212	[3]
Depreciation and amortisation	(1,530)	(1,481)		(1,311)
Other costs	(2,639)	(3,282)		(2,283)
Operating profit	3,493	3,208	[2]	3,225	[3]
Tax	884	(374)	[2]	(427)	[3]
Profit after tax from continuing operations	3,592	1,810	[2]	1,902	[3]
Group’s share of profit (National Grid ownership 50%)	(40)	63	[2]	£ 59	[3]
Quadgas HoldCo Limited
Disclosure of associates [line items]
Non-current assets	16,735	16,047
Current assets	427	299
Non-current liabilities	(11,195)	(10,864)
Current liabilities	(534)	(551)
Net assets	5,433	4,931
Proportion of the Group’s ownership interest in associate	2,119	1,923
Discount for non-controlling interest	(312)	(312)
Impairment charge against investment	(213)	0
Carrying amount of the Group’s interest in associate (National Grid ownership 39%)	1,594	1,611
Revenue	1,468
Depreciation and amortisation	(378)
Other costs	(423)
Operating profit	667
Net interest payable	(272)
Tax	(76)
Profit after tax from continuing operations	319
Group’s share of profit (National Grid ownership 50%)	£ 124
Previously stated | Quadgas HoldCo Limited
Disclosure of associates [line items]
Non-current assets		15,559
Current assets		299
Non-current liabilities		(10,408)
Current liabilities		(519)
Net assets		4,931
Proportion of the Group’s ownership interest in associate		1,923
Discount for non-controlling interest		(312)
Carrying amount of the Group’s interest in associate (National Grid ownership 39%)		£ 1,611
[1]	Comparative amounts have been represented to reflect the reclassification of commodity derivative contracts from trade and other receivables and payables, and from other non-current assets and liabilities to derivative financial assets and derivative financial liabilities (see note 16).
[2]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).
[3]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).

Investments in joint venture134

Investments in joint ventures and associates - Narrative (Details) - GBP (£) £ in Millions	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017		Mar. 31, 2016
Disclosure of joint ventures [line items]
Derivative assets	£ 1,724	£ 1,813	£ 1,724	£ 1,813
Impairment loss on financial assets			213	0
Purchase of interests in investments accounted for using equity method			129	76		£ 116
Share of post-tax results of joint ventures and associates			£ (40)	63	[1]	59	[2]
Proportion of ownership interest in joint venture			50.00%
Capitalisation of shareholder loan to Quadgas HoldCo Limited			£ 69	0
Acquisition of investments			2	0		£ 0
Joint ventures
Disclosure of joint ventures [line items]
Impairment loss on financial assets			0	0
Capital commitments	£ 120	235	120	235
Capitalisation of shareholder loan to Quadgas HoldCo Limited			0	0
BritNed Development Limited
Disclosure of joint ventures [line items]
Share of post-tax results of joint ventures and associates			£ 36	53
Proportion of ownership interest in joint venture	50.00%		50.00%
Quadgas HoldCo Limited
Disclosure of joint ventures [line items]
Proportion of ownership interest in associate, excluding put/call option equity interest			25.00%
Cash proceeds from exercise of put/call options			£ 1,200
Investments in associates	£ 1,594	£ 1,611	£ 1,594	1,611
Proportion of ownership interest in associate		39.00%	39.00%
Receivables due from associates	352	£ 434	£ 352	434
Impairment loss on financial assets			213
Purchase of interests in investments accounted for using equity method				2,100
Equity interests of acquirer		£ 1,600		£ 1,600
Share of post-tax results of joint ventures and associates			£ 124
Proportion of ownership interest in associate, put/call option equity interest			14.00%	14.00%
Loss on purchase price allocation settlement			£ 33
Acquisition of investments			20
Partnership Vehicle with Sunrun
Disclosure of joint ventures [line items]
Purchase of interests in investments accounted for using equity method			38	£ 41
Share of post-tax results of joint ventures and associates			7
Further Acquisition Agreement derivative
Disclosure of joint ventures [line items]
Derivative assets	£ 110		£ 110
[1]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).
[2]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).

Derivative financial instrum135

Derivative financial instruments - Derivative financial instruments (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disclosure of detailed information about financial instruments [line items]
Derivative assets	£ 1,724	£ 1,813
Derivative liabilities	(1,061)	(2,393)
Total	663	(580)
Financing derivatives
Disclosure of detailed information about financial instruments [line items]
Derivative assets	1,545	1,707
Derivative liabilities	(945)	(2,223)
Total	600	(516)
Commodity contract derivatives
Disclosure of detailed information about financial instruments [line items]
Derivative assets	69	106
Derivative liabilities	(116)	(170)
Total	(47)	(64)
Further Acquisition Agreement derivative
Disclosure of detailed information about financial instruments [line items]
Derivative assets	110	0
Derivative liabilities	0	0
Total	£ 110	0
Reclassification of commodity derivative contracts from trade and other receivables to derivative financial assets
Disclosure of detailed information about financial instruments [line items]
Amount of reclassifications or changes in presentation		54
Reclassification of commodity derivative contracts from trade and other payables to derivative financial liabilities
Disclosure of detailed information about financial instruments [line items]
Amount of reclassifications or changes in presentation		93
Reclassification of commodity derivative contracts from other non-current assets to derivative financial assets
Disclosure of detailed information about financial instruments [line items]
Amount of reclassifications or changes in presentation		52
Reclassification of commodity derivative contracts from other non-current liabilities to derivative financial liabilities
Disclosure of detailed information about financial instruments [line items]
Amount of reclassifications or changes in presentation		£ 77
Quadgas HoldCo Limited
Disclosure of detailed information about financial instruments [line items]
Proportion of ownership interest in associate, put/call option equity interest	14.00%	14.00%

Derivative financial instrum136

Derivative financial instruments - Financing derivatives (Details) - GBP (£) £ in Millions	Mar. 31, 2018	Mar. 31, 2017
Disclosure of detailed information about financial instruments [line items]
Derivative notional amount	£ (23,099)	£ (28,576)
Derivative assets	1,724	1,813
Derivative liabilities	(1,061)	(2,393)
Total	663	(580)
Financing derivatives
Disclosure of detailed information about financial instruments [line items]
Derivative assets	1,545	1,707
Derivative liabilities	(945)	(2,223)
Total	600	(516)
Financing Derivatives - Interest Rate Swaps
Disclosure of detailed information about financial instruments [line items]
Derivative notional amount	(8,390)	(9,469)
Derivative assets	678	849
Derivative liabilities	(457)	(657)
Total	221	192
Financing Derivatives - Cross Currency Interest Rate Swaps
Disclosure of detailed information about financial instruments [line items]
Derivative notional amount	(6,925)	(8,631)
Derivative assets	687	676
Derivative liabilities	(207)	(909)
Total	480	(233)
Financing Derivatives - Foreign Exchange Forward Contracts
Disclosure of detailed information about financial instruments [line items]
Derivative notional amount	(5,793)	(8,253)
Derivative assets	174	160
Derivative liabilities	(2)	(113)
Total	172	47
Financing Derivatives - Inflation Linked Swaps
Disclosure of detailed information about financial instruments [line items]
Derivative notional amount	(1,191)	(1,423)
Derivative assets	5	7
Derivative liabilities	(278)	(529)
Total	(273)	(522)
Financing Derivatives - Equity Options
Disclosure of detailed information about financial instruments [line items]
Derivative notional amount	(800)	(800)
Derivative assets	1	15
Derivative liabilities	(1)	(15)
Total	0	0
Less than 1 year | Financing derivatives
Disclosure of detailed information about financial instruments [line items]
Derivative assets	375	192
Derivative liabilities	(325)	(1,054)
Total	50	(862)
Later than one year [member] | Financing derivatives
Disclosure of detailed information about financial instruments [line items]
Derivative assets	1,170	1,515
Derivative liabilities	(620)	(1,169)
Total	550	346
In 1 to 2 years | Financing derivatives
Disclosure of detailed information about financial instruments [line items]
Derivative assets	83	199
Derivative liabilities	(88)	(305)
Total	(5)	(106)
In 2 to 3 years | Financing derivatives
Disclosure of detailed information about financial instruments [line items]
Derivative assets	25	122
Derivative liabilities	(27)	(160)
Total	(2)	(38)
In 3 to 4 years | Financing derivatives
Disclosure of detailed information about financial instruments [line items]
Derivative assets	418	39
Derivative liabilities	(5)	(83)
Total	413	(44)
In 4 to 5 years | Financing derivatives
Disclosure of detailed information about financial instruments [line items]
Derivative assets	12	419
Derivative liabilities	0	(36)
Total	12	383
More than 5 years | Financing derivatives
Disclosure of detailed information about financial instruments [line items]
Derivative assets	632	736
Derivative liabilities	(500)	(585)
Total	132	151
Capital Expenditure | Financing Derivatives - Foreign Exchange Forward Contracts
Disclosure of detailed information about financial instruments [line items]
Total	£ 67	£ 69

Derivative financial instrum137

Derivative financial instruments - Commodity derivatives (Details) £ in Millions	12 Months Ended
	Mar. 31, 2018GBP (£)GWMMBTU	Mar. 31, 2017GBP (£)GWMMBTU
Disclosure of detailed information about financial instruments [line items]
Derivative assets	£ 1,724	£ 1,813
Derivative liabilities	(1,061)	(2,393)
Total	663	(580)
Derivative contractual amount	(23,099)	(28,576)
Commodity contract derivatives
Disclosure of detailed information about financial instruments [line items]
Derivative assets	69	106
Derivative liabilities	(116)	(170)
Total	(47)	(64)
Forward purchases of electricity
Disclosure of detailed information about financial instruments [line items]
Derivative assets	0	0
Derivative liabilities	0	(10)
Total	£ 0	£ (10)
Derivative non-monetary notional amount | GW	0	159
Derivative contractual amount	£ 0	£ 15
Forward purchases of gas
Disclosure of detailed information about financial instruments [line items]
Derivative assets	60	82
Derivative liabilities	(64)	(97)
Total	£ (4)	£ (15)
Derivative non-monetary notional amount | MMBTU	54	54
Derivative contractual amount	£ 96	£ 131
Derivative term	3 years
Electricity capacity
Disclosure of detailed information about financial instruments [line items]
Derivative assets	£ 1	2
Derivative liabilities	0	0
Total	£ 1	£ 2
Derivative non-monetary notional amount | GW	0.6	0.7
Electricity swaps
Disclosure of detailed information about financial instruments [line items]
Derivative assets	£ 7	£ 11
Derivative liabilities	(46)	(61)
Total	£ (39)	£ (50)
Derivative non-monetary notional amount | GW	12,839	12,776
Electricity options
Disclosure of detailed information about financial instruments [line items]
Derivative assets	£ 0	£ 0
Derivative liabilities	(1)	0
Total	£ (1)	£ 0
Derivative non-monetary notional amount | GW	13,897	17,793
Gas swaps
Disclosure of detailed information about financial instruments [line items]
Derivative assets	£ 1	£ 11
Derivative liabilities	(4)	(2)
Total	£ (3)	£ 9
Derivative non-monetary notional amount | MMBTU	100	83
Gas options
Disclosure of detailed information about financial instruments [line items]
Derivative assets	£ 0	£ 0
Derivative liabilities	(1)	0
Total	£ (1)	£ 0
Derivative non-monetary notional amount | MMBTU	7	9
NYMEZ Gas Futures [Member]
Disclosure of detailed information about financial instruments [line items]
Derivative non-monetary notional amount | MMBTU	0	3
Less than 1 year | Commodity contract derivatives
Disclosure of detailed information about financial instruments [line items]
Derivative assets	£ 30	£ 54
Derivative liabilities	(76)	(93)
Total	(46)	(39)
Later than one year [member] | Commodity contract derivatives
Disclosure of detailed information about financial instruments [line items]
Derivative assets	39	52
Derivative liabilities	(40)	(77)
Total	(1)	(25)
In 1 to 2 years | Commodity contract derivatives
Disclosure of detailed information about financial instruments [line items]
Derivative assets	6	8
Derivative liabilities	(17)	(36)
Total	(11)	(28)
In 2 to 3 years | Commodity contract derivatives
Disclosure of detailed information about financial instruments [line items]
Derivative assets	6	7
Derivative liabilities	(11)	(9)
Total	(5)	(2)
In 3 to 4 years | Commodity contract derivatives
Disclosure of detailed information about financial instruments [line items]
Derivative assets	6	6
Derivative liabilities	(3)	(7)
Total	3	(1)
In 4 to 5 years | Commodity contract derivatives
Disclosure of detailed information about financial instruments [line items]
Derivative assets	5	6
Derivative liabilities	(2)	(5)
Total	3	1
More than 5 years | Commodity contract derivatives
Disclosure of detailed information about financial instruments [line items]
Derivative assets	16	25
Derivative liabilities	(7)	(20)
Total	£ 9	£ 5

Derivative financial instrum138

Derivative financial instruments - FWA Derivatives (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disclosure of detailed information about financial instruments [line items]
Total	£ 663	£ (580)
Derivative notional amount	(23,099)	(28,576)
Further Acquisition Agreement derivative
Disclosure of detailed information about financial instruments [line items]
Total	110	0
Derivative notional amount	£ 739	£ 690
Quadgas HoldCo Limited
Disclosure of detailed information about financial instruments [line items]
Proportion of ownership interest in associate, put/call option equity interest	14.00%	14.00%

Derivative financial instrum139

Derivative financial instruments - Hedge accounting (Details) - GBP (£) £ in Millions	Mar. 31, 2018	Mar. 31, 2017
Fair value hedges | Cross-currency interest rate/interest rate swaps
Disclosure of detailed information about hedging instruments [line items]
Financial instruments designated as hedging instruments, at fair value	£ 496	£ 548
Cash flow hedges
Disclosure of detailed information about hedging instruments [line items]
Financial instruments designated as hedging instruments, at fair value	204	(111)
Cash flow hedges | Cross-currency interest rate/interest rate swaps
Disclosure of detailed information about hedging instruments [line items]
Financial instruments designated as hedging instruments, at fair value	138	(180)
Cash flow hedges | Foreign exchange forward contracts
Disclosure of detailed information about hedging instruments [line items]
Financial instruments designated as hedging instruments, at fair value	66	69
Net investment hedges
Disclosure of detailed information about hedging instruments [line items]
Financial instruments designated as hedging instruments, at fair value	(17)	(600)
Net investment hedges | Cross-currency interest rate/interest rate swaps
Disclosure of detailed information about hedging instruments [line items]
Financial instruments designated as hedging instruments, at fair value	(104)	(544)
Net investment hedges | Foreign exchange forward contracts
Disclosure of detailed information about hedging instruments [line items]
Financial instruments designated as hedging instruments, at fair value	£ 87	£ (56)

Derivative financial instrum140

Derivative financial instruments - Not in a formal hedging relationship (Details) - GBP (£) £ in Millions	Mar. 31, 2018	Mar. 31, 2017
Disclosure of detailed information about financial instruments [line items]
Derivatives not in a formal hedge relationship	£ (20)	£ (417)
Cross-currency interest rate/interest rate swaps
Disclosure of detailed information about financial instruments [line items]
Derivatives not in a formal hedge relationship	171	135
Foreign exchange forward contracts
Disclosure of detailed information about financial instruments [line items]
Derivatives not in a formal hedge relationship	19	34
Inflation linked swaps
Disclosure of detailed information about financial instruments [line items]
Derivatives not in a formal hedge relationship	(273)	(522)
Equity options
Disclosure of detailed information about financial instruments [line items]
Derivatives not in a formal hedge relationship	0	0
Commodity contract derivatives
Disclosure of detailed information about financial instruments [line items]
Derivatives not in a formal hedge relationship	(47)	(64)
Further Acquisition Agreement derivative
Disclosure of detailed information about financial instruments [line items]
Derivatives not in a formal hedge relationship	£ 110	£ 0

Inventories and other curren141

Inventories and other current intangibles (Details) - GBP (£) £ in Millions	Mar. 31, 2018	Mar. 31, 2017
Subclassifications of assets, liabilities and equities [abstract]
Fuel stocks	£ 78	£ 101
Raw materials and consumables	190	191
Work in progress	0	8
Current intangible assets – emission allowances	73	103
Inventories and current intangible assets	341	403
Inventory valuation reserve	£ 18	£ 26

Trade and other receivables (De

Trade and other receivables (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disclosure of trade and other receivables [line items]
Trade receivables	£ 1,674	£ 1,591
Accrued income	817	811
Prepayments	229	228
Other receivables	78	98
Trade and other current receivables	2,798	2,728	[1]
Reconciliation of changes in allowance account for credit losses of financial assets [abstract]
Trade receivables past due but not impaired	4,524	9,891
Trade receivables
Reconciliation of changes in allowance account for credit losses of financial assets [abstract]
Allowance account for credit losses of financial assets, beginning balance	424	349
Exchange adjustments	(42)	51
Charge for the year, net of recoveries	36	147
Uncollectible amounts written off against receivables	(109)	(121)
Disposal of UK Gas Distribution	0	(2)
Allowance account for credit losses of financial assets, ending balance	309	424
Financial assets past due but not impaired | Trade receivables
Reconciliation of changes in allowance account for credit losses of financial assets [abstract]
Trade receivables past due but not impaired	475	448
Financial assets past due but not impaired | Trade receivables | Up to 3 months past due
Reconciliation of changes in allowance account for credit losses of financial assets [abstract]
Trade receivables past due but not impaired	271	238
Financial assets past due but not impaired | Trade receivables | 3 to 6 months past due
Reconciliation of changes in allowance account for credit losses of financial assets [abstract]
Trade receivables past due but not impaired	73	67
Financial assets past due but not impaired | Trade receivables | Over 6 months past due
Reconciliation of changes in allowance account for credit losses of financial assets [abstract]
Trade receivables past due but not impaired	£ 131	£ 143
[1]	Comparative amounts have been represented to reflect the reclassification of commodity derivative contracts from trade and other receivables and payables, and from other non-current assets and liabilities to derivative financial assets and derivative financial liabilities (see note 16).

Cash and cash equivalents (Deta

Cash and cash equivalents (Details) - GBP (£) £ in Millions	Mar. 31, 2018	Mar. 31, 2017		Mar. 31, 2016
Cash and cash equivalents [abstract]
Cash at bank	£ 54	£ 199
Short-term deposits	275	940
Cash and cash equivalents	£ 329	£ 1,139	[1]	£ 127
[1]	Comparative amounts have been represented to reflect the reclassification of commodity derivative contracts from trade and other receivables and payables, and from other non-current assets and liabilities to derivative financial assets and derivative financial liabilities (see note 16).

Borrowings - Current and non-cu

Borrowings - Current and non-current borrowings (Details) - GBP (£) £ in Millions	Mar. 31, 2018	Mar. 31, 2017		Mar. 31, 2016
Current
Bank loans	£ 2,020	£ 1,339
Bonds	2,156	2,209
Commercial paper	206	1,881
Finance leases	64	66
Other loans	1	1
Current portion of non-current borrowings	4,447	5,496	[1]
Non-current
Bank loans	2,384	2,343
Bonds	19,418	20,368
Finance leases	207	242
Other loans	169	189
Non-current portion of non-current borrowings	22,178	23,142	[1]
Total borrowings	£ 26,625	£ 28,638		£ 28,344
[1]	Comparative amounts have been represented to reflect the reclassification of commodity derivative contracts from trade and other receivables and payables, and from other non-current assets and liabilities to derivative financial assets and derivative financial liabilities (see note 16).

Borrowings - Borrowings repayme

Borrowings - Borrowings repayment schedule (Details) - GBP (£) £ in Millions	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Disclosure of detailed information about borrowings [line items]
Borrowings	£ 26,625	£ 28,638	£ 28,344
Less than 1 year
Disclosure of detailed information about borrowings [line items]
Borrowings	4,447	5,496
In 1 to 2 years
Disclosure of detailed information about borrowings [line items]
Borrowings	1,694	1,941
In 2 to 3 years
Disclosure of detailed information about borrowings [line items]
Borrowings	2,347	1,821
In 3 to 4 years
Disclosure of detailed information about borrowings [line items]
Borrowings	1,873	2,453
In 4 to 5 years
Disclosure of detailed information about borrowings [line items]
Borrowings	1,469	1,921
Borrowings Paid By Instalments | More than 5 years
Disclosure of detailed information about borrowings [line items]
Borrowings	1,010	1,043
Borrowings Paid By Other Than Instalments | More than 5 years
Disclosure of detailed information about borrowings [line items]
Borrowings	£ 13,785	£ 13,963

Borrowings - Finance lease obli

Borrowings - Finance lease obligations (Details) - GBP (£) £ in Millions	Mar. 31, 2018	Mar. 31, 2017
Disclosure of finance lease and operating lease by lessee [line items]
Minimum finance lease payments payable	£ 313	£ 368
Less: finance charges allocated to future periods	(42)	(60)
Minimum finance lease payments payable, at present value	271	308
Less than 1 year
Disclosure of finance lease and operating lease by lessee [line items]
Minimum finance lease payments payable	64	66
Minimum finance lease payments payable, at present value	64	66
1 to 5 years
Disclosure of finance lease and operating lease by lessee [line items]
Minimum finance lease payments payable	177	213
Minimum finance lease payments payable, at present value	144	174
More than 5 years
Disclosure of finance lease and operating lease by lessee [line items]
Minimum finance lease payments payable	72	89
Minimum finance lease payments payable, at present value	£ 63	£ 68

Borrowings - Narrative (Details

Borrowings - Narrative (Details) - GBP (£) £ in Millions	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Disclosure of fair value measurement of liabilities [line items]
Borrowings	£ 26,625	£ 28,638	£ 28,344
Property, plant and equipment pledged as security	392	440
Collateral received	878	709
Borrowings
Disclosure of fair value measurement of liabilities [line items]
Notional amount	26,363	28,310
At fair value
Disclosure of fair value measurement of liabilities [line items]
Borrowings	30,164	32,239
At fair value | Level 1
Disclosure of fair value measurement of liabilities [line items]
Borrowings	13,018	12,547
At fair value | Level 2
Disclosure of fair value measurement of liabilities [line items]
Borrowings	£ 17,146	£ 19,692

Borrowings - Unaudited commenta

Borrowings - Unaudited commentary (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Disclosure of detailed information about borrowings [line items]
Borrowings	£ 26,625	£ 28,638	£ 28,344
Decrease in borrowings	£ 2,013
Borrowings average maturity	11 years	11 years
Less than 1 year
Disclosure of detailed information about borrowings [line items]
Borrowings	£ 4,447	£ 5,496

Trade and other payables (Detai

Trade and other payables (Details) - GBP (£) £ in Millions	Mar. 31, 2018	Mar. 31, 2017
Subclassifications of assets, liabilities and equities [abstract]
Trade payables	£ 1,977	£ 2,135
Deferred income	440	298
Social security and other taxes	173	136
Other payables	863	776
Trade and other current payables	3,453	3,345	[1]
Customer contributions	£ 85	£ 89
[1]	Comparative amounts have been represented to reflect the reclassification of commodity derivative contracts from trade and other receivables and payables, and from other non-current assets and liabilities to derivative financial assets and derivative financial liabilities (see note 16).

Other non-current liabilitie150

Other non-current liabilities (Details) - GBP (£) £ in Millions	Mar. 31, 2018	Mar. 31, 2017
Subclassifications of assets, liabilities and equities [abstract]
Deferred income	£ 958	£ 1,032
Other payables	359	338
Other non-current liabilities	1,317	1,370	[1]
Customer contributions, non-current	£ 844	£ 839
[1]	Comparative amounts have been represented to reflect the reclassification of commodity derivative contracts from trade and other receivables and payables, and from other non-current assets and liabilities to derivative financial assets and derivative financial liabilities (see note 16).

Pensions and other post reti151

Pensions and other post retirement benefits - Narrative (Details) £ in Millions	12 Months Ended
	Mar. 31, 2018GBP (£)plan	Mar. 31, 2017GBP (£)	Mar. 31, 2016GBP (£)
Disclosure of defined benefit plans [line items]
Defined contribution plan, employer matching contribution percent of gross pay	6.00%
US Pensions
Disclosure of defined benefit plans [line items]
Number of defined benefit plans | plan	4
UK Pensions
Disclosure of defined benefit plans [line items]
Payment upon credit rating trigger, percent of agencies	66.67%
Payment upon credit rating trigger, consecutive period	40 days
Maximum payment upon triggers	£ 500
National Grid UK Pension Scheme - Section A | UK Pensions
Disclosure of defined benefit plans [line items]
Contribution rate to meet future benefit accrual, percentage of pensionable earnings		51.80%
Security agreement value	315
Payment upon credit rating trigger	72
National Grid UK Pension Scheme - Section B | UK Pensions
Disclosure of defined benefit plans [line items]
Contribution rate to meet future benefit accrual, percentage of pensionable earnings		51.40%
Security agreement value	179
Payment upon credit rating trigger	65
National Grid UK Pension Scheme - Section B | UK Pensions | Time Period, 30 September 2017 - 30 September 2022
Disclosure of defined benefit plans [line items]
Contributions payable per year		£ 32
National Grid UK Pension Scheme - Section B | UK Pensions | Time Period, 30 September 2023
Disclosure of defined benefit plans [line items]
Contributions payable per year		£ 8
NGEG of ESPS | UK Pensions
Disclosure of defined benefit plans [line items]
Contribution rate to meet future benefit accrual, percentage of pensionable earnings			40.70%
Security agreement value	£ 250
Notice period trigger	12 months
Maximum payment upon triggers	£ 500
NGEG of ESPS | UK Pensions | Time Period 2016/17 - 31 March 2027
Disclosure of defined benefit plans [line items]
Contributions payable per year			£ 48

Pensions and other post reti152

Pensions and other post retirement benefits - Actuarial assumptions (Details) - year	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
UK Pensions
Disclosure of defined benefit plans [line items]
Discount rate - past service	2.60%	2.40%	3.30%
Discount rate - future service	2.65%	2.65%	3.30%
Rate of increase in salaries	3.40%	3.50%	3.20%
Rate of increase in RPI, past service	3.15%	3.20%	2.90%
Rate of increase in RPI, future service	3.10%	3.15%	2.90%
Assumed life expectations for a retiree age 65 today: Males	22.3	22.9	22.8
Assumed life expectations for a retiree age 65 today: Females	23.9	24.7	25.2
Assumed life expectations for a retiree age 65 today: In 20 years: Males	23.7	25.1	25.1
Assumed life expectations for a retiree age 65 today: In 20 years: Females	25.5	27.1	27.6
Weighted average duration of defined benefit obligation	16
Rate of increase in salaries - prior service	2.20%	2.20%	2.10%
Rate of increase in salaries - future service	2.15%	2.20%	2.10%
Duration of scheme liabilities, future service	25 years
Duration of scheme liabilities, past service	17 years
US Pensions
Disclosure of defined benefit plans [line items]
Discount rate	4.00%	4.25%	4.25%
Rate of increase in salaries	3.50%	3.50%	3.50%
Assumed life expectations for a retiree age 65 today: Males	22	21.9	21.8
Assumed life expectations for a retiree age 65 today: Females	24.2	24.1	24
Assumed life expectations for a retiree age 65 today: In 20 years: Males	23.6	23.6	23.5
Assumed life expectations for a retiree age 65 today: In 20 years: Females	25.8	25.7	25.6
Weighted average duration of defined benefit obligation	13
US other post-retirement benefits
Disclosure of defined benefit plans [line items]
Discount rate	4.00%	4.25%	4.25%
Rate of increase in salaries	3.50%	3.50%	3.50%
Initial healthcare cost trend rate	7.50%	7.00%	7.50%
Ultimate healthcare cost trend rate	4.50%	4.50%	4.50%
Weighted average duration of defined benefit obligation	15

Pensions and other post reti153

Pensions and other post retirement benefits - Results of the most recent actuarial valuations (Details) - GBP (£) £ in Millions	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Disclosure of defined benefit plans [line items]
Fair value of plan assets	£ 23,858
Market value of scheme assets at latest valuation	24,121
Funding surplus/(deficit)	(263)	£ (1,933)	£ (2,585)
UK Pensions
Disclosure of defined benefit plans [line items]
Fair value of plan assets	15,330	15,489	19,401
Market value of scheme assets at latest valuation	14,226
Funding surplus/(deficit)	£ 1,104	(156)	(15)
National Grid UK Pension Scheme - Section A | UK Pensions
Disclosure of defined benefit plans [line items]
Fair value of plan assets		6,716
Market value of scheme assets at latest valuation		£ 6,627
Market value as percentage of benefits		101.00%
Funding surplus/(deficit)		£ 89
Funding surplus/(deficit) net of tax		74
National Grid UK Pension Scheme - Section B | UK Pensions
Disclosure of defined benefit plans [line items]
Fair value of plan assets		5,849
Market value of scheme assets at latest valuation		£ 6,057
Market value as percentage of benefits		97.00%
Funding surplus/(deficit)		£ (208)
Funding surplus/(deficit) net of tax		£ (173)
NGEG of ESPS | UK Pensions
Disclosure of defined benefit plans [line items]
Fair value of plan assets			2,553
Market value of scheme assets at latest valuation			£ 3,053
Market value as percentage of benefits			84.00%
Funding surplus/(deficit)			£ (500)
Funding surplus/(deficit) net of tax			£ (415)

Pensions and other post reti154

Pensions and other post retirement benefits - Amounts recognized in statement of financial position (Details) - GBP (£) £ in Millions	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Disclosure of defined benefit plans [line items]
Defined benefit obligation, at present value	£ (24,121)
Fair value of plan assets	23,858
Funding surplus/(deficit)	(263)	£ (1,933)	£ (2,585)
Liabilities	(1,672)	(2,536)	(2,995)
Assets	1,409	603	410
Pensions | Funded obligations
Disclosure of defined benefit plans [line items]
Defined benefit obligation, at present value	(23,747)	(25,890)	(28,648)
Fair value of plan assets	23,858	24,375	26,434
Funding surplus/(deficit)	111	(1,515)	(2,214)
Pensions | Unfunded obligations
Disclosure of defined benefit plans [line items]
Defined benefit obligation, at present value	(307)	(340)	(304)
UK Pensions
Disclosure of defined benefit plans [line items]
Defined benefit obligation, at present value	(14,226)
Fair value of plan assets	15,330	15,489	19,401
Funding surplus/(deficit)	1,104	(156)	(15)
Liabilities	(74)	(536)	(300)
Assets	1,178	380	285
UK Pensions | Funded obligations
Disclosure of defined benefit plans [line items]
Defined benefit obligation, at present value	(14,152)	(15,565)	(19,341)
Fair value of plan assets	15,330	15,489	19,401
Funding surplus/(deficit)	1,178	(76)	60
UK Pensions | Unfunded obligations
Disclosure of defined benefit plans [line items]
Defined benefit obligation, at present value	(74)	(80)	(75)
Other Post-employment Liabilities	£ 0	£ 0	£ 0
UK Pensions | Active members
Disclosure of defined benefit plans [line items]
Defined benefit obligation, member concentration percentage	10.00%	12.00%	12.00%
UK Pensions | Deferred members
Disclosure of defined benefit plans [line items]
Defined benefit obligation, member concentration percentage	18.00%	19.00%	18.00%
UK Pensions | Pensioner members
Disclosure of defined benefit plans [line items]
Defined benefit obligation, member concentration percentage	72.00%	69.00%	70.00%
US Pensions
Disclosure of defined benefit plans [line items]
Fair value of plan assets	£ 6,030	£ 6,322	£ 5,136
Funding surplus/(deficit)	(552)	(728)	(1,009)
Liabilities	(783)	(951)	(1,134)
Assets	231	223	125
US Pensions | Funded obligations
Disclosure of defined benefit plans [line items]
Defined benefit obligation, at present value	(6,349)	(6,790)	(5,916)
Fair value of plan assets	6,030	6,322	5,136
Funding surplus/(deficit)	(319)	(468)	(780)
US Pensions | Unfunded obligations
Disclosure of defined benefit plans [line items]
Defined benefit obligation, at present value	(233)	(260)	(229)
Other Post-employment Liabilities	£ 0	£ 0	£ 0
US Pensions | Active members
Disclosure of defined benefit plans [line items]
Defined benefit obligation, member concentration percentage	38.00%	38.00%	39.00%
US Pensions | Deferred members
Disclosure of defined benefit plans [line items]
Defined benefit obligation, member concentration percentage	8.00%	9.00%	9.00%
US Pensions | Pensioner members
Disclosure of defined benefit plans [line items]
Defined benefit obligation, member concentration percentage	54.00%	53.00%	52.00%
US other post-retirement benefits
Disclosure of defined benefit plans [line items]
Other Post-employment Liabilities	£ (67)	£ (78)	£ (67)
Fair value of plan assets	2,498	2,564	1,897
Funding surplus/(deficit)	(815)	(1,049)	(1,561)
Liabilities	(815)	(1,049)	(1,561)
Assets	0	0	0
US other post-retirement benefits | Funded obligations
Disclosure of defined benefit plans [line items]
Defined benefit obligation, at present value	(3,246)	(3,535)	(3,391)
Fair value of plan assets	2,498	2,564	1,897
Funding surplus/(deficit)	(748)	(971)	(1,494)
US other post-retirement benefits | Unfunded obligations
Disclosure of defined benefit plans [line items]
Defined benefit obligation, at present value	£ 0	£ 0	£ 0
US other post-retirement benefits | Active members
Disclosure of defined benefit plans [line items]
Defined benefit obligation, member concentration percentage	38.00%	39.00%	41.00%
US other post-retirement benefits | Deferred members
Disclosure of defined benefit plans [line items]
Defined benefit obligation, member concentration percentage	0.00%	0.00%	0.00%
US other post-retirement benefits | Pensioner members
Disclosure of defined benefit plans [line items]
Defined benefit obligation, member concentration percentage	62.00%	61.00%	59.00%

Pensions and other post reti155

Pensions and other post retirement benefits - Amounts recognised in the income statement and statement of other comprehensive income (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Included within operating costs
Administration costs	£ 16	£ 16	£ 16
Included within payroll costs
Current service cost	193	232	221
Past service cost – augmentations	1	1	3
Past service credit – redundancies	(1)	(1)	(1)
Special termination benefit cost – redundancies	9	7	11
Total included within payroll costs	202	239	234
Included within finance income and costs
Net interest cost	65	105	112
Included within gain on disposal of discontinued operations
Administration costs	0	5	2
Disposal of UK Gas Distribution	0	34	0
Total included within gain on disposal of discontinued operations	0	39	2
Total included in income statement	283	399	364
Remeasurement gains of pension assets and post-retirement benefit obligations	1,313	348	539
Exchange adjustments	175	(345)	(81)
Total included in the statement of other comprehensive income	1,488	3	458
UK Pensions
Included within operating costs
Administration costs	6	6	9
Included within payroll costs
Current service cost	49	76	74
Past service cost – augmentations	1	1	3
Past service credit – redundancies	(1)	(1)	(1)
Special termination benefit cost – redundancies	9	7	11
Total included within payroll costs	58	83	87
Included within finance income and costs
Net interest cost	3	0	18
Included within gain on disposal of discontinued operations
Administration costs	0	5	2
Disposal of UK Gas Distribution	0	34	0
Total included within gain on disposal of discontinued operations	0	39	2
Total included in income statement	67	128	116
Remeasurement gains of pension assets and post-retirement benefit obligations	1,177	(541)	534
Exchange adjustments	0	0	0
Total included in the statement of other comprehensive income	1,177	(541)	534
UK Pensions | Discontinued operations
Included within operating costs
Administration costs	0	(1)	(1)
Included within payroll costs
Total included within payroll costs	0	35	28
Included within finance income and costs
Net interest cost	0	(2)	(1)
Included within gain on disposal of discontinued operations
Remeasurement gains of pension assets and post-retirement benefit obligations	0	(75)	129
US Pensions
Included within operating costs
Administration costs	9	9	6
Included within payroll costs
Current service cost	98	103	95
Past service cost – augmentations	0	0	0
Past service credit – redundancies	0	0	0
Special termination benefit cost – redundancies	0	0	0
Total included within payroll costs	98	103	95
Included within finance income and costs
Net interest cost	27	43	36
Included within gain on disposal of discontinued operations
Administration costs	0	0	0
Disposal of UK Gas Distribution	0	0	0
Total included within gain on disposal of discontinued operations	0	0	0
Total included in income statement	134	155	137
Remeasurement gains of pension assets and post-retirement benefit obligations	27	319	(67)
Exchange adjustments	75	(140)	(33)
Total included in the statement of other comprehensive income	102	179	(100)
US other post-retirement benefits
Included within operating costs
Administration costs	1	1	1
Included within payroll costs
Current service cost	46	53	52
Past service cost – augmentations	0	0	0
Past service credit – redundancies	0	0	0
Special termination benefit cost – redundancies	0	0	0
Total included within payroll costs	46	53	52
Included within finance income and costs
Net interest cost	35	62	58
Included within gain on disposal of discontinued operations
Administration costs	0	0	0
Disposal of UK Gas Distribution	0	0	0
Total included within gain on disposal of discontinued operations	0	0	0
Total included in income statement	82	116	111
Remeasurement gains of pension assets and post-retirement benefit obligations	109	570	72
Exchange adjustments	100	(205)	(48)
Total included in the statement of other comprehensive income	£ 209	£ 365	£ 24

Pensions and other post reti156

Pensions and other post retirement benefits - Reconciliation of the net defined benefit liability (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Disclosure of net defined benefit liability (asset) [line items]
Net defined benefit liability (asset), beginning balance	£ (1,933)	£ (2,585)	£ (3,258)
Changes in net defined benefit liability (asset) [abstract]
Cost recognised in the income statement	(283)	(399)	(364)
Remeasurement and foreign exchange effects recognised in the statement of other comprehensive income	1,488	3	458
Employer contributions	475	1,073	587
Other movements	(10)	(25)	(8)
Net defined benefit liability (asset), ending balance	(263)	(1,933)	(2,585)
UK Pensions
Disclosure of net defined benefit liability (asset) [line items]
Net defined benefit liability (asset), beginning balance	(156)	(15)	(672)
Changes in net defined benefit liability (asset) [abstract]
Cost recognised in the income statement	(67)	(128)	(116)
Remeasurement and foreign exchange effects recognised in the statement of other comprehensive income	1,177	(541)	534
Employer contributions	150	528	239
Other movements	0	0	0
Net defined benefit liability (asset), ending balance	1,104	(156)	(15)
US Pensions
Disclosure of net defined benefit liability (asset) [line items]
Net defined benefit liability (asset), beginning balance	(728)	(1,009)	(1,003)
Changes in net defined benefit liability (asset) [abstract]
Cost recognised in the income statement	(134)	(155)	(137)
Remeasurement and foreign exchange effects recognised in the statement of other comprehensive income	102	179	(100)
Employer contributions	208	257	231
Other movements	0	0	0
Net defined benefit liability (asset), ending balance	(552)	(728)	(1,009)
US other post-retirement benefits
Disclosure of net defined benefit liability (asset) [line items]
Net defined benefit liability (asset), beginning balance	(1,049)	(1,561)	(1,583)
Changes in net defined benefit liability (asset) [abstract]
Cost recognised in the income statement	(82)	(116)	(111)
Remeasurement and foreign exchange effects recognised in the statement of other comprehensive income	209	365	24
Employer contributions	117	288	117
Other movements	(10)	(25)	(8)
Net defined benefit liability (asset), ending balance	£ (815)	£ (1,049)	£ (1,561)

Pensions and other post reti157

Pensions and other post retirement benefits - Change in the present value of defined benefit obligations (including unfunded obligations) (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Disclosure of operating segments [line items]
Net defined benefit liability (asset), beginning balance	£ (1,933)	£ (2,585)	£ (3,258)
Changes in net defined benefit liability (asset) [abstract]
Current service cost	193	232	221
Net interest cost	65	105	112
Past service credit – redundancies	(1)	(1)	(1)
Special termination benefit cost – redundancies	9	7	11
Past service cost – augmentations	1	1	3
Disposal of UK Gas Distribution	0	34	0
Exchange adjustments	175	(345)	(81)
Net defined benefit liability (asset), ending balance	(263)	(1,933)	(2,585)
UK Pensions
Disclosure of operating segments [line items]
Net defined benefit liability (asset), beginning balance	(156)	(15)	(672)
Changes in net defined benefit liability (asset) [abstract]
Current service cost	49	76	74
Net interest cost	3	0	18
Past service credit – redundancies	(1)	(1)	(1)
Special termination benefit cost – redundancies	9	7	11
Past service cost – augmentations	1	1	3
Disposal of UK Gas Distribution	0	34	0
Exchange adjustments	0	0	0
Net defined benefit liability (asset), ending balance	1,104	(156)	(15)
US Pensions
Disclosure of operating segments [line items]
Net defined benefit liability (asset), beginning balance	(728)	(1,009)	(1,003)
Changes in net defined benefit liability (asset) [abstract]
Current service cost	98	103	95
Net interest cost	27	43	36
Past service credit – redundancies	0	0	0
Special termination benefit cost – redundancies	0	0	0
Past service cost – augmentations	0	0	0
Disposal of UK Gas Distribution	0	0	0
Exchange adjustments	75	(140)	(33)
Net defined benefit liability (asset), ending balance	(552)	(728)	(1,009)
US other post-retirement benefits
Disclosure of operating segments [line items]
Net defined benefit liability (asset), beginning balance	(1,049)	(1,561)	(1,583)
Changes in net defined benefit liability (asset) [abstract]
Current service cost	46	53	52
Net interest cost	35	62	58
Past service credit – redundancies	0	0	0
Special termination benefit cost – redundancies	0	0	0
Past service cost – augmentations	0	0	0
Disposal of UK Gas Distribution	0	0	0
Exchange adjustments	100	(205)	(48)
Net defined benefit liability (asset), ending balance	(815)	(1,049)	(1,561)
Present value of defined benefit obligation
Disclosure of operating segments [line items]
Net defined benefit liability (asset), beginning balance	(26,230)	(28,952)	(29,592)
Changes in net defined benefit liability (asset) [abstract]
Current service cost	(193)	(232)	(221)
Net interest cost	(775)	(1,057)	(1,026)
Actuarial (losses)/gains – experience	(100)	166	659
Actuarial gains – demographic assumptions	671	225	0
Actuarial gains/(losses) – financial assumptions	174	(3,377)	218
Past service credit – redundancies	1	1	1
Special termination benefit cost – redundancies	(9)	(7)	(11)
Past service cost – augmentations	(1)	(1)	(3)
Medicare subsidy received	(21)	(14)	(15)
Disposal of UK Gas Distribution	0	6,970	0
Employee contributions	(1)	(1)	(2)
Benefits paid	1,285	1,443	1,348
Exchange adjustments	1,145	(1,394)	(308)
Net defined benefit liability (asset), ending balance	(24,054)	(26,230)	(28,952)
Present value of defined benefit obligation | UK Pensions
Disclosure of operating segments [line items]
Net defined benefit liability (asset), beginning balance	(15,645)	(19,416)	(20,125)
Changes in net defined benefit liability (asset) [abstract]
Current service cost	(49)	(76)	(74)
Net interest cost	(366)	(615)	(649)
Actuarial (losses)/gains – experience	(95)	106	552
Actuarial gains – demographic assumptions	565	214	0
Actuarial gains/(losses) – financial assumptions	604	(3,751)	0
Past service credit – redundancies	1	1	1
Special termination benefit cost – redundancies	(9)	(7)	(11)
Past service cost – augmentations	(1)	(1)	(3)
Medicare subsidy received	0	0	0
Disposal of UK Gas Distribution	0	6,970	0
Employee contributions	(1)	(1)	(2)
Benefits paid	770	931	895
Exchange adjustments	0	0	0
Net defined benefit liability (asset), ending balance	(14,226)	(15,645)	(19,416)
Present value of defined benefit obligation | US Pensions
Disclosure of operating segments [line items]
Net defined benefit liability (asset), beginning balance	(7,050)	(6,145)	(6,055)
Changes in net defined benefit liability (asset) [abstract]
Current service cost	(98)	(103)	(95)
Net interest cost	(273)	(285)	(242)
Actuarial (losses)/gains – experience	(38)	(2)	15
Actuarial gains – demographic assumptions	30	2	0
Actuarial gains/(losses) – financial assumptions	(279)	37	120
Past service credit – redundancies	0	0	0
Special termination benefit cost – redundancies	0	0	0
Past service cost – augmentations	0	0	0
Medicare subsidy received	0	0	0
Disposal of UK Gas Distribution	0	0	0
Employee contributions	0	0	0
Benefits paid	362	349	310
Exchange adjustments	764	(903)	(198)
Net defined benefit liability (asset), ending balance	(6,582)	(7,050)	(6,145)
Present value of defined benefit obligation | US other post-retirement benefits
Disclosure of operating segments [line items]
Net defined benefit liability (asset), beginning balance	(3,535)	(3,391)	(3,412)
Changes in net defined benefit liability (asset) [abstract]
Current service cost	(46)	(53)	(52)
Net interest cost	(136)	(157)	(135)
Actuarial (losses)/gains – experience	33	62	92
Actuarial gains – demographic assumptions	76	9	0
Actuarial gains/(losses) – financial assumptions	(151)	337	98
Past service credit – redundancies	0	0	0
Special termination benefit cost – redundancies	0	0	0
Past service cost – augmentations	0	0	0
Medicare subsidy received	(21)	(14)	(15)
Disposal of UK Gas Distribution	0	0	0
Employee contributions	0	0	0
Benefits paid	153	163	143
Exchange adjustments	381	(491)	(110)
Net defined benefit liability (asset), ending balance	£ (3,246)	£ (3,535)	£ (3,391)

Pensions and other post reti158

Pensions and other post retirement benefits - Change in fair value of plan assets (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Disclosure of net defined benefit liability (asset) [line items]
Net defined benefit liability (asset), beginning balance	£ (1,933)	£ (2,585)	£ (3,258)
Changes in net defined benefit liability (asset) [abstract]
Interest income	65	105	112
Employer contributions	475	1,073	587
Exchange adjustments	175	(345)	(81)
Assets transferred on disposal of UK Gas Distribution	0	34	0
Net defined benefit liability (asset), ending balance	(263)	(1,933)	(2,585)
Actual return on plan assets	1,278	4,286	576
Expected contributions to plans in the following year	363	491	686
Plan assets
Disclosure of net defined benefit liability (asset) [line items]
Net defined benefit liability (asset), beginning balance	24,375	26,434	26,408
Changes in net defined benefit liability (asset) [abstract]
Interest income	710	952	914
Return on plan assets in excess of/(less than) interest	568	3,334	(338)
Administration costs	(16)	(21)	(18)
Employer contributions	475	1,073	587
Employee contributions	1	1	2
Benefits paid	(1,285)	(1,443)	(1,348)
Exchange adjustments	(970)	1,049	227
Assets transferred on disposal of UK Gas Distribution	0	(7,004)	0
Net defined benefit liability (asset), ending balance	23,858	24,375	26,434
UK Pensions
Disclosure of net defined benefit liability (asset) [line items]
Net defined benefit liability (asset), beginning balance	(156)	(15)	(672)
Changes in net defined benefit liability (asset) [abstract]
Interest income	3	0	18
Employer contributions	150	528	239
Exchange adjustments	0	0	0
Assets transferred on disposal of UK Gas Distribution	0	34	0
Net defined benefit liability (asset), ending balance	1,104	(156)	(15)
Actual return on plan assets	466	3,505	613
Expected contributions to plans in the following year	140	128	228
UK Pensions | Plan assets
Disclosure of net defined benefit liability (asset) [line items]
Net defined benefit liability (asset), beginning balance	15,489	19,401	19,453
Changes in net defined benefit liability (asset) [abstract]
Interest income	363	615	631
Return on plan assets in excess of/(less than) interest	103	2,890	(18)
Administration costs	(6)	(11)	(11)
Employer contributions	150	528	239
Employee contributions	1	1	2
Benefits paid	(770)	(931)	(895)
Exchange adjustments	0	0	0
Assets transferred on disposal of UK Gas Distribution	0	(7,004)	0
Net defined benefit liability (asset), ending balance	15,330	15,489	19,401
US Pensions
Disclosure of net defined benefit liability (asset) [line items]
Net defined benefit liability (asset), beginning balance	(728)	(1,009)	(1,003)
Changes in net defined benefit liability (asset) [abstract]
Interest income	27	43	36
Employer contributions	208	257	231
Exchange adjustments	75	(140)	(33)
Assets transferred on disposal of UK Gas Distribution	0	0	0
Net defined benefit liability (asset), ending balance	(552)	(728)	(1,009)
Actual return on plan assets	560	524	4
Expected contributions to plans in the following year	221	229	220
US Pensions | Plan assets
Disclosure of net defined benefit liability (asset) [line items]
Net defined benefit liability (asset), beginning balance	6,322	5,136	5,052
Changes in net defined benefit liability (asset) [abstract]
Interest income	246	242	206
Return on plan assets in excess of/(less than) interest	314	282	(202)
Administration costs	(9)	(9)	(6)
Employer contributions	208	257	231
Employee contributions	0	0	0
Benefits paid	(362)	(349)	(310)
Exchange adjustments	(689)	763	165
Assets transferred on disposal of UK Gas Distribution	0	0	0
Net defined benefit liability (asset), ending balance	6,030	6,322	5,136
US other post-retirement benefits
Disclosure of net defined benefit liability (asset) [line items]
Net defined benefit liability (asset), beginning balance	(1,049)	(1,561)	(1,583)
Changes in net defined benefit liability (asset) [abstract]
Interest income	35	62	58
Employer contributions	117	288	117
Exchange adjustments	100	(205)	(48)
Assets transferred on disposal of UK Gas Distribution	0	0	0
Net defined benefit liability (asset), ending balance	(815)	(1,049)	(1,561)
Actual return on plan assets	252	257	(41)
Expected contributions to plans in the following year	2	134	238
US other post-retirement benefits | Plan assets
Disclosure of net defined benefit liability (asset) [line items]
Net defined benefit liability (asset), beginning balance	2,564	1,897	1,903
Changes in net defined benefit liability (asset) [abstract]
Interest income	101	95	77
Return on plan assets in excess of/(less than) interest	151	162	(118)
Administration costs	(1)	(1)	(1)
Employer contributions	117	288	117
Employee contributions	0	0	0
Benefits paid	(153)	(163)	(143)
Exchange adjustments	(281)	286	62
Assets transferred on disposal of UK Gas Distribution	0	0	0
Net defined benefit liability (asset), ending balance	£ 2,498	£ 2,564	£ 1,897

Pensions and other post reti159

Pensions and other post retirement benefits - Asset allocation percentages (Details)	Mar. 31, 2018
UK Pensions
Disclosure of fair value of plan assets [line items]
Equities	14.60%
Property	6.30%
Diversified alternatives	5.10%
Liability matching assets	7.70%
Other	3.80%
Total	100.00%
US Pensions
Disclosure of fair value of plan assets [line items]
Equities	42.00%
Property	4.60%
Diversified alternatives	10.30%
Liability matching assets	0.00%
Other	2.10%
Total	100.00%
US other post-retirement benefits
Disclosure of fair value of plan assets [line items]
Equities	60.90%
Property	0.00%
Diversified alternatives	12.20%
Liability matching assets	0.00%
Other	5.50%
Total	100.00%
Corporate bonds | UK Pensions
Disclosure of fair value of plan assets [line items]
Debt instruments	25.80%
Corporate bonds | US Pensions
Disclosure of fair value of plan assets [line items]
Debt instruments	24.80%
Corporate bonds | US other post-retirement benefits
Disclosure of fair value of plan assets [line items]
Debt instruments	1.00%
Government securities | UK Pensions
Disclosure of fair value of plan assets [line items]
Debt instruments	36.70%
Government securities | US Pensions
Disclosure of fair value of plan assets [line items]
Debt instruments	16.20%
Government securities | US other post-retirement benefits
Disclosure of fair value of plan assets [line items]
Debt instruments	20.40%
UK | UK Pensions
Disclosure of fair value of plan assets [line items]
Total	57.80%
UK | US Pensions
Disclosure of fair value of plan assets [line items]
Total	1.20%
UK | US other post-retirement benefits
Disclosure of fair value of plan assets [line items]
Total	0.20%
US | UK Pensions
Disclosure of fair value of plan assets [line items]
Total	18.40%
US | US Pensions
Disclosure of fair value of plan assets [line items]
Total	88.50%
US | US other post-retirement benefits
Disclosure of fair value of plan assets [line items]
Total	91.80%
Other Countries | UK Pensions
Disclosure of fair value of plan assets [line items]
Total	23.70%
Other Countries | US Pensions
Disclosure of fair value of plan assets [line items]
Total	10.30%
Other Countries | US other post-retirement benefits
Disclosure of fair value of plan assets [line items]
Total	8.00%

Pensions and other post reti160

Pensions and other post retirement benefits - Asset allocations (Details) - GBP (£) £ in Millions	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Disclosure of fair value of plan assets [line items]
Fair value of plan assets	£ 23,858
UK Pensions
Disclosure of fair value of plan assets [line items]
Equities	2,233	£ 3,220	£ 4,234
Property	963	798	1,171
Diversified alternatives	789	878	664
Liability matching assets	1,174	1,162	1,020
Other	593	499	652
Fair value of plan assets	15,330	15,489	19,401
Ordinary shares of National Grid including in fair value of plan assets	0	2	7
Bonds issued by subsidiary included in fair value of plan assets	0	0	70
UK Pensions | Quoted
Disclosure of fair value of plan assets [line items]
Equities	1,420	2,624	3,272
Property	129	90	90
Diversified alternatives	99	250	159
Liability matching assets	1,174	1,162	1,020
Other	211	63	649
Fair value of plan assets	12,611	13,121	16,850
UK Pensions | Unquoted
Disclosure of fair value of plan assets [line items]
Equities	813	596	962
Property	834	708	1,081
Diversified alternatives	690	628	505
Liability matching assets	0	0	0
Other	382	436	3
Fair value of plan assets	2,719	2,368	2,551
US Pensions
Disclosure of fair value of plan assets [line items]
Equities	2,531	2,613	2,133
Property	279	335	276
Diversified alternatives	619	642	497
Other	124	122	82
Fair value of plan assets	6,030	6,322	5,136
US Pensions | Quoted
Disclosure of fair value of plan assets [line items]
Equities	577	698	625
Property	0	0	0
Diversified alternatives	198	209	163
Other	20	31	0
Fair value of plan assets	2,294	2,940	2,453
US Pensions | Unquoted
Disclosure of fair value of plan assets [line items]
Equities	1,954	1,915	1,508
Property	279	335	276
Diversified alternatives	421	433	334
Other	104	91	82
Fair value of plan assets	3,736	3,382	2,683
US other post-retirement benefits
Disclosure of fair value of plan assets [line items]
Equities	1,522	1,567	1,134
Diversified alternatives	305	315	226
Other	137	142	109
Fair value of plan assets	2,498	2,564	1,897
US other post-retirement benefits | Quoted
Disclosure of fair value of plan assets [line items]
Equities	412	405	281
Diversified alternatives	161	166	122
Other	0	0	0
Fair value of plan assets	1,105	1,110	830
US other post-retirement benefits | Unquoted
Disclosure of fair value of plan assets [line items]
Equities	1,110	1,162	853
Diversified alternatives	144	149	104
Other	137	142	109
Fair value of plan assets	1,393	1,454	1,067
Corporate bonds | UK Pensions
Disclosure of fair value of plan assets [line items]
Debt instruments	3,949	3,526	5,601
Corporate bonds | UK Pensions | Quoted
Disclosure of fair value of plan assets [line items]
Debt instruments	3,949	3,526	5,601
Corporate bonds | UK Pensions | Unquoted
Disclosure of fair value of plan assets [line items]
Debt instruments	0	0	0
Corporate bonds | US Pensions
Disclosure of fair value of plan assets [line items]
Debt instruments	1,498	1,667	1,437
Corporate bonds | US Pensions | Quoted
Disclosure of fair value of plan assets [line items]
Debt instruments	1,085	1,130	954
Corporate bonds | US Pensions | Unquoted
Disclosure of fair value of plan assets [line items]
Debt instruments	413	537	483
Corporate bonds | US other post-retirement benefits
Disclosure of fair value of plan assets [line items]
Debt instruments	24	19	38
Corporate bonds | US other post-retirement benefits | Quoted
Disclosure of fair value of plan assets [line items]
Debt instruments	24	19	37
Corporate bonds | US other post-retirement benefits | Unquoted
Disclosure of fair value of plan assets [line items]
Debt instruments	0	0	1
Government securities | UK Pensions
Disclosure of fair value of plan assets [line items]
Debt instruments	5,629	5,406	6,059
Government securities | UK Pensions | Quoted
Disclosure of fair value of plan assets [line items]
Debt instruments	5,629	5,406	6,059
Government securities | UK Pensions | Unquoted
Disclosure of fair value of plan assets [line items]
Debt instruments	0	0	0
Government securities | US Pensions
Disclosure of fair value of plan assets [line items]
Debt instruments	979	943	711
Government securities | US Pensions | Quoted
Disclosure of fair value of plan assets [line items]
Debt instruments	414	872	711
Government securities | US Pensions | Unquoted
Disclosure of fair value of plan assets [line items]
Debt instruments	565	71	0
Government securities | US other post-retirement benefits
Disclosure of fair value of plan assets [line items]
Debt instruments	510	521	390
Government securities | US other post-retirement benefits | Quoted
Disclosure of fair value of plan assets [line items]
Debt instruments	508	520	390
Government securities | US other post-retirement benefits | Unquoted
Disclosure of fair value of plan assets [line items]
Debt instruments	£ 2	£ 1	£ 0

Provisions Schedule of changes

Provisions Schedule of changes in provisions (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disclosure of other provisions [line items]
Other provisions, beginning balance	£ 2,588	£ 1,719
Exchange adjustments	(195)	188
Additions	74	1,015
Unused amounts reversed	(107)	(86)
Unwinding of discount	75	73
Utilised	(268)	(227)
Disposal of UK gas distribution		(94)
Transfers	(115)
Other provisions, ending balance	2,052	2,588
Current	273	416	[1]
Non-current	1,779	2,172	[1]
Transferred to trade and other payables	115	0
Environmental
Disclosure of other provisions [line items]
Other provisions, beginning balance	1,721	1,169
Exchange adjustments	(158)	137
Additions	27	572
Unused amounts reversed	(45)	(75)
Unwinding of discount	61	60
Utilised	(75)	(110)
Disposal of UK gas distribution		(32)
Transfers	0
Other provisions, ending balance	1,531	1,721
Decommissioning
Disclosure of other provisions [line items]
Other provisions, beginning balance	221	141
Exchange adjustments	(9)	13
Additions	2	107
Unused amounts reversed	(19)	0
Unwinding of discount	5	0
Utilised	(6)	(19)
Disposal of UK gas distribution		(21)
Transfers	0
Other provisions, ending balance	194	221
Restructuring
Disclosure of other provisions [line items]
Other provisions, beginning balance	17	30
Exchange adjustments	0	0
Additions	10	16
Unused amounts reversed	(6)	(5)
Unwinding of discount	1	1
Utilised	(7)	(25)
Disposal of UK gas distribution		0
Transfers	(12)
Other provisions, ending balance	3	17
Emissions
Disclosure of other provisions [line items]
Other provisions, beginning balance	32	18
Exchange adjustments	(2)	2
Additions	12	12
Unused amounts reversed	0	0
Unwinding of discount	0	0
Utilised	(34)	0
Disposal of UK gas distribution		0
Transfers	0
Other provisions, ending balance	8	32
Other
Disclosure of other provisions [line items]
Other provisions, beginning balance	597	361
Exchange adjustments	(26)	36
Additions	23	308
Unused amounts reversed	(37)	(6)
Unwinding of discount	8	12
Utilised	(146)	(73)
Disposal of UK gas distribution		(41)
Transfers	(103)
Other provisions, ending balance	316	£ 597
Discontinued operations
Disclosure of other provisions [line items]
Unused amounts reversed	(16)
Utilised	£ (77)
[1]	Comparative amounts have been represented to reflect the reclassification of commodity derivative contracts from trade and other receivables and payables, and from other non-current assets and liabilities to derivative financial assets and derivative financial liabilities (see note 16).

Provisions Environmental provis

Provisions Environmental provision (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Disclosure of other provisions [line items]
Other provisions	£ 2,052	£ 2,588	£ 1,719
Discounted	£ 1,645	£ 1,889
Real discount rate	1.00%	2.00%
UK
Disclosure of other provisions [line items]
Discounted	£ 235	£ 270
Real discount rate	1.00%	1.00%
US
Disclosure of other provisions [line items]
Discounted	£ 1,410	£ 1,619
Real discount rate	1.00%	1.00%
Environmental
Disclosure of other provisions [line items]
Other provisions	£ 1,531	£ 1,721	£ 1,169
Environmental | UK
Disclosure of other provisions [line items]
Other provisions	213	242
Environmental | US
Disclosure of other provisions [line items]
Other provisions	£ 1,318	£ 1,479

Provisions Narrative (Details)

Provisions Narrative (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Disclosure of other provisions [line items]
Other provisions	£ 2,052	£ 2,588	£ 1,719
Real discount rate	1.00%	2.00%
Decommissioning
Disclosure of other provisions [line items]
Other provisions	£ 194	£ 221	141
Other
Disclosure of other provisions [line items]
Other provisions	316	597	£ 361
Voluntary Distribution
Disclosure of other provisions [line items]
Other provisions	150	117
Business set up and restructuring cost
Disclosure of other provisions [line items]
Other provisions	50	143
Liability for claims and claims adjustment
Disclosure of other provisions [line items]
Other provisions	152	166
Obligations associated with investments in joint ventures and associates
Disclosure of other provisions [line items]
Other provisions	13	13
Later than seventy five years and not later than eighty years | Decommissioning provision, ARO
Disclosure of other provisions [line items]
Other provisions	71	78
Later than seven years and not later than ten years | Provision for decommissioning, demolition of gas holders
Disclosure of other provisions [line items]
Other provisions	104	107
Later than twenty years and not later than twenty-five years | Onerous lease commitments and rates payable on surplus properties
Disclosure of other provisions [line items]
Other provisions	£ 48	£ 84

Share capital - Schedule of sha

Share capital - Schedule of share capital (Details) £ / shares in Units, £ in Millions, shares in Millions		May 22, 2017£ / shares	Mar. 31, 2018GBP (£)shares		Mar. 31, 2017GBP (£)shares		Mar. 31, 2016GBP (£)shares	May 21, 2017£ / shares
Disclosure of classes of share capital [line items]
Equity, beginning balance			£ 20,384	[1]	£ 13,565		£ 11,974
Issued during the year in lieu of dividends	[2]		0		0		(1)
Equity, ending balance			£ 18,848	[3]	£ 20,384	[1]	£ 13,565
Share consolidation ratio		0.9167
Par value per share (per share) | £ / shares		£ 0.1139535						£ 0.1243129
Share capital
Disclosure of classes of share capital [line items]
Issued capital (in shares), beginning balance | shares			3,943		3,924
Issued during period, in lieu of dividends (in shares) | shares			23		19
Effect of share consolidation (in shares) | shares			(328)
Issued capital (in shares), ending balance | shares			3,638		3,943		3,924
Equity, beginning balance			£ 449		£ 447		£ 443
Issued during the year in lieu of dividends	[2]		3		2		4
Equity, ending balance			£ 452	[3]	£ 449		£ 447
[1]	Comparative amounts have been represented to reflect the reclassification of commodity derivative contracts from trade and other receivables and payables, and from other non-current assets and liabilities to derivative financial assets and derivative financial liabilities (see note 16).
[2]	Included within the share premium account are costs associated with scrip dividends.
[3]	Refer to note 7 for the effect of the share consolidation and the special dividend.

Share capital - Narrative (Deta

Share capital - Narrative (Details) - GBP (£) £ in Millions, shares in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Share Capital, Reserves And Other Equity Interest [Abstract]
Treasury stock (in shares)	283	193
Treasury shares, market value	£ 2,270	£ 1,958
Treasury stock acquired (in shares)	114	20
Purchase of treasury shares	£ 1,017	£ 189	£ 267
Purchase of treasury shares, nominal value	£ 14	£ 2
Shares repurchased as percent of ordinary shares	3.10%	0.50%
Treasury shares, gifted (in shares)	3	3
Treasury stock, reissued (in shares)	5	3
Shares reissued to employee share schemes as percent of ordinary shares	0.20%	0.20%
Treasury shares, reissued, nominal value	£ 1	£ 1
Proceeds from issue of treasury shares	33	18	16
Purchase of own shares, employee share trusts	£ 5	£ 6	£ 6
Maximum number of treasury shares held during the year (in shares)	283	194
Treasury shares as percent of ordinary shares	7.80%	4.90%
Treasury shares, nominal value	£ 35	£ 22

Other equity reserves (Details)

Other equity reserves (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018		Mar. 31, 2017		Mar. 31, 2016
Disclosure of reserves within equity [line items]
Other equity reserves, beginning balance	£ (3,987)	[1]	£ (4,523)		£ (4,682)
Exchange adjustments	(504)		346		69
Net gains taken to equity	(11)		45		93
Transferred to profit or loss	(76)		202		29
Share of net gains taken to equity of associates	5		0		0
Tax	33		(57)		(32)
Other equity reserves, ending balance	(4,540)		(3,987)	[1]	(4,523)
Other comprehensive income, before tax, hedges of net investments in foreign operations	(1,304)		1,364		275
Exchange adjustments	800		(1,018)		(206)
Translation
Disclosure of reserves within equity [line items]
Other equity reserves, beginning balance	894		548		479
Exchange adjustments	(504)		346		69
Other equity reserves, ending balance	390		894		548
Cash flow hedge
Disclosure of reserves within equity [line items]
Other equity reserves, beginning balance	103		(45)		(109)
Net gains taken to equity	19		(36)		50
Transferred to profit or loss	(3)		227		29
Share of net gains taken to equity of associates	5
Tax	4		(43)		(15)
Other equity reserves, ending balance	128		103		(45)
Available- for-sale
Disclosure of reserves within equity [line items]
Other equity reserves, beginning balance	162		120		94
Net gains taken to equity	(30)		81		43
Transferred to profit or loss	(73)		(25)
Tax	29		(14)		(17)
Other equity reserves, ending balance	88		162		120
Capital redemption
Disclosure of reserves within equity [line items]
Other equity reserves, beginning balance	19		19		19
Other equity reserves, ending balance	19		19		19
Merger
Disclosure of reserves within equity [line items]
Other equity reserves, beginning balance	(5,165)		(5,165)		(5,165)
Other equity reserves, ending balance	£ (5,165)		£ (5,165)		£ (5,165)
[1]	Comparative amounts have been represented to reflect the reclassification of commodity derivative contracts from trade and other receivables and payables, and from other non-current assets and liabilities to derivative financial assets and derivative financial liabilities (see note 16).

Other equity reserves Narrative

Other equity reserves Narrative (Details) £ in Millions	12 Months Ended
	Mar. 31, 2018GBP (£)
Disclosure of reserves within equity [abstract]
Cash flow hedge gain loss to be reclassified within 12 months	£ 23
Cash flow hedge gain loss to be reclassified within twelve months relating to capital expenditures	£ 20

Net debt Reconciliation of net

Net debt Reconciliation of net cash flow to movement in net debt (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Subclassifications of assets, liabilities and equities [abstract]
(Decrease)/increase in cash and cash equivalents	£ (807)	£ 984	£ 4
(Decrease)/increase in financial investments	(5,953)	5,675	391
Decrease/(increase) in borrowings and related derivatives	1,209	(3,715)	(1,100)
Net interest paid on the components of net debt2	808	1,955	810
Change in debt resulting from cash flows	(4,743)	4,899	105
Changes in fair value of financial assets and liabilities and exchange movements	2,098	(2,273)	(515)
Net interest charge on the components of net debt	(1,017)	(2,401)	(913)
Net debt deconsolidated on disposal of subsidiary	0	5,890	0
Other non-cash movements	(66)	(64)	(87)
Movement in net debt (net of related derivative financial instruments) in the year	(3,728)	6,051	(1,410)
Net debt (net of related derivative financial instruments) at start of year	(19,274)	(25,325)	(23,915)
Net debt (net of related derivative financial instruments) at end of year	£ (23,002)	£ (19,274)	£ (25,325)

Net debt Composition of net deb

Net debt Composition of net debt (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015
Subclassifications of assets, liabilities and equities [abstract]
Cash, cash equivalents and financial investments	£ 3,023	£ 9,880	£ 3,125
Borrowings and bank overdrafts	(26,625)	(28,638)	(28,344)
Derivatives	600	(516)	(106)
Net debt position	(23,002)	(19,274)	(25,325)	£ (23,915)
Financing derivatives relating to purchase of property, plant and equipment, classified as investing activities	12	18	(16)
Interest shareholder loan	£ 27	£ 0	£ 0

Net debt Rollforward of changes

Net debt Rollforward of changes in net debt (Details) - GBP (£)	12 Months Ended
	Mar. 31, 2018		Mar. 31, 2017		Mar. 31, 2016
Increase (Decrease) In Cash And Cash Equivalents [Roll Forward]
Cash and cash equivalents, beginning balance	£ 1,139,000,000	[1]	£ 127,000,000
Cash flow, cash and cash equivalents excluding bank overdrafts	(807,000,000)		1,001,000,000
Fair value gains and losses and exchange movements, cash and cash equivalents excluding bank overdrafts	(3,000,000)		16,000,000
Disposal, cash and cash equivalents excluding bank overdrafts			(5,000,000)
Cash and cash equivalents, ending balance	329,000,000		1,139,000,000	[1]	£ 127,000,000
Increase (Decrease) In Bank Overdrafts [Roll Forward]
Bank overdrafts, beginning balance	0		(3,000,000)
Cash flow, bank overdrafts	0		(17,000,000)
Disposal, bank overdrafts			20,000,000
Bank overdrafts, ending balance	0		0		(3,000,000)
Increase (Decrease) In Net Cash And Cash Equivalents, Net Of Bank Overdrafts [Roll Forward]
Cash and cash equivalents net bank overdrafts, beginning balance	1,139,000,000		124,000,000
Net (decrease)/increase in cash and cash equivalents	(807,000,000)		984,000,000		4,000,000
Disposal, cash and cash equivalents			15,000,000
Exchange movements	(3,000,000)		16,000,000		4,000,000
Cash and cash equivalents net bank overdrafts, ending balance	329,000,000		1,139,000,000		124,000,000
Increase (Decreases) In Financial Investments [Roll Forward]
Financial investments, beginning balance	8,741,000,000	[1]	2,998,000,000
Cash flow, financial investments	(5,983,000,000)		5,624,000,000
Fair value gains and losses and exchange movements, financial investments	(149,000,000)		141,000,000
Interest income/(charges), financing derivatives	85,000,000		53,000,000
Disposal, financial investments			(75,000,000)
Financial investments, ending balance	2,694,000,000		8,741,000,000	[1]	2,998,000,000
Increase (Decrease) In Borrowings [Roll Forward]
Borrowings, beginning balance	(28,638,000,000)		(28,341,000,000)
Cash flow, borrowings	2,108,000,000		(2,196,000,000)
Fair value gains and losses and exchange movements, borrowings	1,088,000,000		(1,527,000,000)
interest income/(charges), borrowings	(1,117,000,000)		(2,221,000,000)
Other non-cash movements, borrowings	(66,000,000)		(294,000,000)
Disposal, borrowings			5,941,000,000
Borrowings, ending balance	(26,625,000,000)		(28,638,000,000)		(28,341,000,000)
Increase (Decrease) In Financing Derivatives [Roll Forward]
Financing derivatives, beginning balance	(516,000,000)		(106,000,000)
Cash flow, financing derivatives	(61,000,000)		487,000,000
Fair value gains and losses and exchange movements, financing derivatives	1,162,000,000		(903,000,000)
Interest income/(charges), financing derivatives	15,000,000		(233,000,000)
Other non-cash movements, financing derivatives	0		230,000,000
Disposal, financing derivatives			9,000,000
Financing derivatives, ending balance	600,000,000		(516,000,000)		(106,000,000)
Increase (Decrease) In Net Debt [Roll Forward]
Net debt (net of related derivative financial instruments) at start of year	(19,274,000,000)		(25,325,000,000)		(23,915,000,000)
Change in debt resulting from cash flows	(4,743,000,000)		4,899,000,000		105,000,000
Changes in fair value of financial assets and liabilities and exchange movements	2,098,000,000		(2,273,000,000)		(515,000,000)
Net interest charge on the components of net debt	(1,017,000,000)		(2,401,000,000)		(913,000,000)
Other non-cash movements	(66,000,000)		(64,000,000)
Disposal	0		5,890,000,000		0
Net debt (net of related derivative financial instruments) at end of year	(23,002,000,000)		(19,274,000,000)		(25,325,000,000)
Interest payable	197,000,000		210,000,000		243,000,000
Exceptional items
Increase (Decrease) In Net Debt [Roll Forward]
Change in debt resulting from cash flows	(231,000,000)		(1,052,000,000)		0
Net interest charge on the components of net debt	3,000,000		£ (1,313,000,000)		£ 0
Non-current assets
Increase (Decrease) In Financing Derivatives [Roll Forward]
Financing derivatives, ending balance	1,170,000,000
Increase (Decrease) In Net Debt [Roll Forward]
Net debt (net of related derivative financial instruments) at end of year	1,170,000,000
Current assets
Increase (Decrease) In Cash And Cash Equivalents [Roll Forward]
Cash and cash equivalents, ending balance	329,000,000
Increase (Decrease) In Bank Overdrafts [Roll Forward]
Bank overdrafts, ending balance	0
Increase (Decrease) In Net Cash And Cash Equivalents, Net Of Bank Overdrafts [Roll Forward]
Cash and cash equivalents net bank overdrafts, ending balance	329,000,000
Increase (Decreases) In Financial Investments [Roll Forward]
Financial investments, ending balance	2,694,000,000
Increase (Decrease) In Financing Derivatives [Roll Forward]
Financing derivatives, ending balance	375,000,000
Increase (Decrease) In Net Debt [Roll Forward]
Net debt (net of related derivative financial instruments) at end of year	3,398,000,000
Current liabilities
Increase (Decrease) In Borrowings [Roll Forward]
Borrowings, ending balance	(4,447,000,000)
Increase (Decrease) In Financing Derivatives [Roll Forward]
Financing derivatives, ending balance	(620,000,000)
Increase (Decrease) In Net Debt [Roll Forward]
Net debt (net of related derivative financial instruments) at end of year	(5,067,000,000)
Non-current liabilities
Increase (Decrease) In Borrowings [Roll Forward]
Borrowings, ending balance	(22,178,000,000)
Increase (Decrease) In Financing Derivatives [Roll Forward]
Financing derivatives, ending balance	(325,000,000)
Increase (Decrease) In Net Debt [Roll Forward]
Net debt (net of related derivative financial instruments) at end of year	£ (22,503,000,000)
[1]	Comparative amounts have been represented to reflect the reclassification of commodity derivative contracts from trade and other receivables and payables, and from other non-current assets and liabilities to derivative financial assets and derivative financial liabilities (see note 16).

Commitments and contingencie171

Commitments and contingencies (Details) - GBP (£) £ in Millions	Mar. 31, 2018	Mar. 31, 2017
Disclosure of finance lease and operating lease by lessee [line items]
Contracted for but not provided	£ 1,843	£ 1,913
Operating lease commitments	443	619
Energy purchase commitments	5,328	5,699
Estimated financial effect of contingent liabilities	2,669	2,780
Expected future minimum sublease payments receivable under non-cancellable subleases	42	15
Less than 1 year
Disclosure of finance lease and operating lease by lessee [line items]
Operating lease commitments	68	95
Energy purchase commitments	1,237	1,325
In 1 to 2 years
Disclosure of finance lease and operating lease by lessee [line items]
Operating lease commitments	44	82
Energy purchase commitments	700	744
In 2 to 3 years
Disclosure of finance lease and operating lease by lessee [line items]
Operating lease commitments	40	58
Energy purchase commitments	563	587
In 3 to 4 years
Disclosure of finance lease and operating lease by lessee [line items]
Operating lease commitments	37	56
Energy purchase commitments	449	507
In 4 to 5 years
Disclosure of finance lease and operating lease by lessee [line items]
Operating lease commitments	35	54
Energy purchase commitments	410	436
More than 5 years
Disclosure of finance lease and operating lease by lessee [line items]
Operating lease commitments	219	274
Energy purchase commitments	1,969	2,100
Guarantee of sublease for US property (expires 2040)
Disclosure of finance lease and operating lease by lessee [line items]
Estimated financial effect of contingent liabilities	178	225
Guarantees of certain obligations of Grain LNG Import Terminal (expire up to 2028)
Disclosure of finance lease and operating lease by lessee [line items]
Estimated financial effect of contingent liabilities	46	100
Guarantees of certain obligations for construction of HVDC West Coast Link (expires 2018)
Disclosure of finance lease and operating lease by lessee [line items]
Estimated financial effect of contingent liabilities	213	281
Guarantees of certain obligations of Nemo Link Limited (various expiry dates)
Disclosure of finance lease and operating lease by lessee [line items]
Estimated financial effect of contingent liabilities	63	140
Guarantees of certain obligations of National Grid North Sea Link Limited (various expiry dates)3
Disclosure of finance lease and operating lease by lessee [line items]
Estimated financial effect of contingent liabilities	1,009	1,059
Guarantees of certain obligations of St William Homes LLP (various expiry dates)4
Disclosure of finance lease and operating lease by lessee [line items]
Estimated financial effect of contingent liabilities	98	147
Guarantees of certain obligations for construction of IFA 2 SAS (expected expiry 2021)3
Disclosure of finance lease and operating lease by lessee [line items]
Estimated financial effect of contingent liabilities	729	354
Other guarantees and letters of credit (various expiry dates)
Disclosure of finance lease and operating lease by lessee [line items]
Estimated financial effect of contingent liabilities	333	474
Guarantees to both joint ventures and EPC contractors regarding the construction of interconnectors [Member]
Disclosure of finance lease and operating lease by lessee [line items]
Estimated financial effect of contingent liabilities	£ 739	£ 555

Related party transactions - Re

Related party transactions - Related party transactions table (Details) - GBP (£) £ in Millions	3 Months Ended	12 Months Ended
	Jun. 30, 2017	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Pension plan
Disclosure of transactions between related parties [line items]
Revenue from sale of goods and services, related party transactions		£ 3	£ 3	£ 3
Joint ventures
Disclosure of transactions between related parties [line items]
Revenue from sale of goods and services, related party transactions		14	78	9
Purchases of goods and services, related party transactions		135	168	183
Amounts receivable		160	64	7
Amounts payable		0	84	96
Interest income		4	0	0
Dividends received		43	75	48
Associates
Disclosure of transactions between related parties [line items]
Revenue from sale of goods and services, related party transactions		220	0	4
Purchases of goods and services, related party transactions		160	169	83
Amounts receivable		376	457	0
Amounts payable		17	27	7
Interest income		27	0	0
Dividends received		170	24	£ 24
Quadgas HoldCo Limited
Disclosure of transactions between related parties [line items]
Other income, sale of UK gas distribution business		54
Receivables due from associates		352	434
Quadgas HoldCo Limited | Associates
Disclosure of transactions between related parties [line items]
Dividends received		144	0
Nemo Link Limited
Disclosure of transactions between related parties [line items]
Receivables due from associates		130	£ 61
St William Homes LLP
Disclosure of transactions between related parties [line items]
Revenue from sale of goods and services, related party transactions	£ 68	£ 5

Related party transactions - Na

Related party transactions - Narrative (Details)	Sep. 30, 2017USD ($)
Key management personnel of entity or parent
Disclosure of transactions between related parties [line items]
Receivables due from related parties	$ 70,767

Financial risk management - Tr

Financial risk management - Treasury credit risk (Details) £ in Millions	Mar. 31, 2018GBP (£)
Triple ‘A’ G7 sovereign entities (AAA)
Disclosure of credit risk exposure [line items]
Maximum limit	£ 1,853
Triple ‘A’ vehicles (AAA)
Disclosure of credit risk exposure [line items]
Maximum limit	500
Triple ‘A’ range institutions and non G7 sovereign entities (AAA)
Disclosure of credit risk exposure [line items]
Maximum limit	1,011
Double ‘A’ G7 sovereign entities (AA)
Disclosure of credit risk exposure [line items]
Maximum limit	1,685
Bottom of range | Double ‘A’ range institutions (AA)
Disclosure of credit risk exposure [line items]
Maximum limit	843
Bottom of range | Single ‘A’ range institutions (A)
Disclosure of credit risk exposure [line items]
Maximum limit	337
Top of range | Double ‘A’ range institutions (AA)
Disclosure of credit risk exposure [line items]
Maximum limit	674
Top of range | Single ‘A’ range institutions (A)
Disclosure of credit risk exposure [line items]
Maximum limit	236
Long-term limit £m | Triple ‘A’ G7 sovereign entities (AAA)
Disclosure of credit risk exposure [line items]
Maximum limit	927
Long-term limit £m | Triple ‘A’ vehicles (AAA)
Disclosure of credit risk exposure [line items]
Maximum limit	0
Long-term limit £m | Triple ‘A’ range institutions and non G7 sovereign entities (AAA)
Disclosure of credit risk exposure [line items]
Maximum limit	506
Long-term limit £m | Double ‘A’ G7 sovereign entities (AA)
Disclosure of credit risk exposure [line items]
Maximum limit	843
Long-term limit £m | Bottom of range | Double ‘A’ range institutions (AA)
Disclosure of credit risk exposure [line items]
Maximum limit	337
Long-term limit £m | Bottom of range | Single ‘A’ range institutions (A)
Disclosure of credit risk exposure [line items]
Maximum limit	118
Long-term limit £m | Top of range | Double ‘A’ range institutions (AA)
Disclosure of credit risk exposure [line items]
Maximum limit	421
Long-term limit £m | Top of range | Single ‘A’ range institutions (A)
Disclosure of credit risk exposure [line items]
Maximum limit	£ 169

Financial risk management - Off

Financial risk management - Offsetting financial assets and liabilities (Details) - Credit Risk - GBP (£) £ in Millions	Mar. 31, 2018	Mar. 31, 2017
Assets
Gross carrying amounts £m	£ 1,724	£ 1,813
Gross amounts offset £m	0	0
Net amount presented in statement of financial position £m	1,724	1,813
Related amounts available to be offset but not offset in statement of financial position
Financial instruments £m	(539)	(743)
Cash collateral received/ pledged £m	(772)	(692)
Net amount £m	413	378
Liabilities
Gross carrying amounts £m	(1,061)	(2,393)
Gross amounts offset £m	0	0
Net amount presented in statement of financial position £m	(1,061)	(2,393)
Related amounts available to be offset but not offset in statement of financial position
Financial instruments £m	539	743
Cash collateral received/ pledged £m	333	1,248
Net amount £m	(189)	(402)
Assets (Liabilities)
Gross carrying amounts £m	663	(580)
Gross amounts offset £m	0	0
Net amount presented in statement of financial position £m	663	(580)
Related amounts available to be offset but not offset in statement of financial position
Financial instruments £m	0	0
Cash collateral received/ pledged £m	(439)	556
Net amount £m	224	(24)
Financing derivatives
Liabilities
Gross carrying amounts £m	(945)	(2,223)
Gross amounts offset £m	0	0
Net amount presented in statement of financial position £m	(945)	(2,223)
Related amounts available to be offset but not offset in statement of financial position
Financial instruments £m	523	718
Cash collateral received/ pledged £m	326	1,230
Net amount £m	(96)	(275)
Commodity contract derivatives
Liabilities
Gross carrying amounts £m	(116)	(170)
Gross amounts offset £m	0	0
Net amount presented in statement of financial position £m	(116)	(170)
Related amounts available to be offset but not offset in statement of financial position
Financial instruments £m	16	25
Cash collateral received/ pledged £m	7	18
Net amount £m	(93)	(127)
Financing derivatives
Assets
Gross carrying amounts £m	1,545	1,707
Gross amounts offset £m	0	0
Net amount presented in statement of financial position £m	1,545	1,707
Related amounts available to be offset but not offset in statement of financial position
Financial instruments £m	(523)	(718)
Cash collateral received/ pledged £m	(772)	(692)
Net amount £m	250	297
Commodity contract derivatives
Assets
Gross carrying amounts £m	69	106
Gross amounts offset £m	0	0
Net amount presented in statement of financial position £m	69	106
Related amounts available to be offset but not offset in statement of financial position
Financial instruments £m	(16)	(25)
Cash collateral received/ pledged £m	0	0
Net amount £m	53	£ 81
FAA derivative
Assets
Gross carrying amounts £m	110
Gross amounts offset £m	0
Net amount presented in statement of financial position £m	110
Related amounts available to be offset but not offset in statement of financial position
Financial instruments £m	0
Cash collateral received/ pledged £m	0
Net amount £m	£ 110

Financial risk management - Liq

Financial risk management - Liquidity risk (Details) - Liquidity risk - GBP (£) £ in Millions	Mar. 31, 2018	Mar. 31, 2017
Disclosure of maturity analysis for non-derivative financial liabilities [abstract]
Borrowings, excluding finance lease liabilities	£ (26,089)	£ (28,001)
Interest payments on borrowings	(14,948)	(15,119)
Finance lease liabilities	(313)	(368)
Other non interest-bearing liabilities	(3,199)	(3,249)
Derivative financial liabilities
Financial liabilities	(44,507)	(47,864)
Derivative contracts – receipts
Derivative financial liabilities
Derivative contracts – receipts	2,050	2,338
Derivative contracts – payments
Derivative financial liabilities
Derivative contracts – receipts	1,870	3,424
Commodity contract derivatives
Derivative financial liabilities
Commodity contract derivatives	(138)	(41)
Less than 1 year
Disclosure of maturity analysis for non-derivative financial liabilities [abstract]
Borrowings, excluding finance lease liabilities	(4,099)	(5,142)
Interest payments on borrowings	(730)	(767)
Finance lease liabilities	(60)	(66)
Other non interest-bearing liabilities	(2,840)	(2,989)
Derivative financial liabilities
Financial liabilities	(7,630)	(9,959)
Less than 1 year | Derivative contracts – receipts
Derivative financial liabilities
Derivative contracts – receipts	1,069	571
Less than 1 year | Derivative contracts – payments
Derivative financial liabilities
Derivative contracts – receipts	890	1,551
Less than 1 year | Commodity contract derivatives
Derivative financial liabilities
Commodity contract derivatives	(80)	(15)
In 1 to 2 years
Disclosure of maturity analysis for non-derivative financial liabilities [abstract]
Borrowings, excluding finance lease liabilities	(1,642)	(1,864)
Interest payments on borrowings	(692)	(707)
Finance lease liabilities	(60)	(58)
Other non interest-bearing liabilities	(359)	(260)
Derivative financial liabilities
Financial liabilities	(2,448)	(2,905)
In 1 to 2 years | Derivative contracts – receipts
Derivative financial liabilities
Derivative contracts – receipts	601	961
In 1 to 2 years | Derivative contracts – payments
Derivative financial liabilities
Derivative contracts – receipts	263	959
In 1 to 2 years | Commodity contract derivatives
Derivative financial liabilities
Commodity contract derivatives	(33)	(18)
In 2 to 3 years
Disclosure of maturity analysis for non-derivative financial liabilities [abstract]
Borrowings, excluding finance lease liabilities	(2,325)	(1,750)
Interest payments on borrowings	(629)	(670)
Finance lease liabilities	(45)	(61)
Other non interest-bearing liabilities	0	0
Derivative financial liabilities
Financial liabilities	(3,083)	(2,221)
In 2 to 3 years | Derivative contracts – receipts
Derivative financial liabilities
Derivative contracts – receipts	130	572
In 2 to 3 years | Derivative contracts – payments
Derivative financial liabilities
Derivative contracts – receipts	188	304
In 2 to 3 years | Commodity contract derivatives
Derivative financial liabilities
Commodity contract derivatives	(26)	(8)
More than 3 years
Disclosure of maturity analysis for non-derivative financial liabilities [abstract]
Borrowings, excluding finance lease liabilities	(18,023)	(19,245)
Interest payments on borrowings	(12,897)	(12,975)
Finance lease liabilities	(148)	(183)
Other non interest-bearing liabilities	0	0
Derivative financial liabilities
Financial liabilities	(31,346)	(32,779)
More than 3 years | Derivative contracts – receipts
Derivative financial liabilities
Derivative contracts – receipts	250	234
More than 3 years | Derivative contracts – payments
Derivative financial liabilities
Derivative contracts – receipts	529	610
More than 3 years | Commodity contract derivatives
Derivative financial liabilities
Commodity contract derivatives	£ 1	£ 0

Financial risk management - Int

Financial risk management - Interest rate risk (Details) - Interest rate risk - GBP (£) £ in Millions	Mar. 31, 2018	Mar. 31, 2017
Disclosure of financial instruments by type of interest rate [line items]
Risk exposure associated with instruments sharing characteristic	£ (23,002)	£ (19,274)
Fixed rate £m
Disclosure of financial instruments by type of interest rate [line items]
Risk exposure associated with instruments sharing characteristic	(14,378)	(15,273)
Floating rate £m
Disclosure of financial instruments by type of interest rate [line items]
Risk exposure associated with instruments sharing characteristic	(1,501)	3,188
Inflation linked £m
Disclosure of financial instruments by type of interest rate [line items]
Risk exposure associated with instruments sharing characteristic	(7,188)	(7,195)
Other £m
Disclosure of financial instruments by type of interest rate [line items]
Risk exposure associated with instruments sharing characteristic	65	6
Cash and cash equivalents
Disclosure of financial instruments by type of interest rate [line items]
Risk exposure associated with instruments sharing characteristic	329	1,139
Cash and cash equivalents | Fixed rate £m
Disclosure of financial instruments by type of interest rate [line items]
Risk exposure associated with instruments sharing characteristic	0	940
Cash and cash equivalents | Floating rate £m
Disclosure of financial instruments by type of interest rate [line items]
Risk exposure associated with instruments sharing characteristic	302	199
Cash and cash equivalents | Inflation linked £m
Disclosure of financial instruments by type of interest rate [line items]
Risk exposure associated with instruments sharing characteristic	0	0
Cash and cash equivalents | Other £m
Disclosure of financial instruments by type of interest rate [line items]
Risk exposure associated with instruments sharing characteristic	27	0
Financial investments
Disclosure of financial instruments by type of interest rate [line items]
Risk exposure associated with instruments sharing characteristic	2,694	8,741
Financial investments | Fixed rate £m
Disclosure of financial instruments by type of interest rate [line items]
Risk exposure associated with instruments sharing characteristic	31	44
Financial investments | Floating rate £m
Disclosure of financial instruments by type of interest rate [line items]
Risk exposure associated with instruments sharing characteristic	2,625	8,691
Financial investments | Inflation linked £m
Disclosure of financial instruments by type of interest rate [line items]
Risk exposure associated with instruments sharing characteristic	0	0
Financial investments | Other £m
Disclosure of financial instruments by type of interest rate [line items]
Risk exposure associated with instruments sharing characteristic	38	6
Borrowings
Disclosure of financial instruments by type of interest rate [line items]
Risk exposure associated with instruments sharing characteristic	(26,625)	(28,638)
Borrowings | Fixed rate £m
Disclosure of financial instruments by type of interest rate [line items]
Risk exposure associated with instruments sharing characteristic	(16,144)	(17,681)
Borrowings | Floating rate £m
Disclosure of financial instruments by type of interest rate [line items]
Risk exposure associated with instruments sharing characteristic	(3,191)	(3,917)
Borrowings | Inflation linked £m
Disclosure of financial instruments by type of interest rate [line items]
Risk exposure associated with instruments sharing characteristic	(7,290)	(7,040)
Borrowings | Other £m
Disclosure of financial instruments by type of interest rate [line items]
Risk exposure associated with instruments sharing characteristic	0	0
Pre-derivative position
Disclosure of financial instruments by type of interest rate [line items]
Risk exposure associated with instruments sharing characteristic	(23,602)	(18,758)
Pre-derivative position | Fixed rate £m
Disclosure of financial instruments by type of interest rate [line items]
Risk exposure associated with instruments sharing characteristic	(16,113)	(16,697)
Pre-derivative position | Floating rate £m
Disclosure of financial instruments by type of interest rate [line items]
Risk exposure associated with instruments sharing characteristic	(264)	4,973
Pre-derivative position | Inflation linked £m
Disclosure of financial instruments by type of interest rate [line items]
Risk exposure associated with instruments sharing characteristic	(7,290)	(7,040)
Pre-derivative position | Other £m
Disclosure of financial instruments by type of interest rate [line items]
Risk exposure associated with instruments sharing characteristic	65	6
Financing derivatives
Disclosure of financial instruments by type of interest rate [line items]
Risk exposure associated with instruments sharing characteristic	600	(516)
Financing derivatives | Fixed rate £m
Disclosure of financial instruments by type of interest rate [line items]
Risk exposure associated with instruments sharing characteristic	1,735	1,424
Financing derivatives | Floating rate £m
Disclosure of financial instruments by type of interest rate [line items]
Risk exposure associated with instruments sharing characteristic	(1,237)	(1,785)
Financing derivatives | Inflation linked £m
Disclosure of financial instruments by type of interest rate [line items]
Risk exposure associated with instruments sharing characteristic	102	(155)
Financing derivatives | Other £m
Disclosure of financial instruments by type of interest rate [line items]
Risk exposure associated with instruments sharing characteristic	£ 0	£ 0

Financial risk management - Cur

Financial risk management - Currency risk (Details) - Currency risk - GBP (£) £ in Millions	Mar. 31, 2018	Mar. 31, 2017
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	£ (23,002)	£ (19,274)
Cash and cash equivalents
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	329	1,139
Financial investments
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	2,694	8,741
Borrowings
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	(26,625)	(28,638)
Pre-derivative position
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	(23,602)	(18,758)
Financing derivatives
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	600	(516)
Trade and other receivables
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	1,781	1,743
Trade and other payables
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	(2,917)	(3,004)
Other non-current liabilities
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	(398)	(415)
Sterling £m
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	(5,399)	(855)
Sterling £m | Cash and cash equivalents
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	294	1,110
Sterling £m | Financial investments
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	1,471	6,824
Sterling £m | Borrowings
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	(10,912)	(11,099)
Sterling £m | Pre-derivative position
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	(9,147)	(3,165)
Sterling £m | Financing derivatives
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	3,748	2,310
Sterling £m | Trade and other receivables
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	253	83
Sterling £m | Trade and other payables
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	(1,124)	(1,209)
Sterling £m | Other non-current liabilities
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	(144)	(100)
Euro £m
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	70	966
Euro £m | Cash and cash equivalents
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	2	0
Euro £m | Financial investments
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	69	98
Euro £m | Borrowings
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	(3,794)	(5,373)
Euro £m | Pre-derivative position
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	(3,723)	(5,275)
Euro £m | Financing derivatives
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	3,793	6,241
Euro £m | Trade and other receivables
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	0	0
Euro £m | Trade and other payables
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	0	0
Euro £m | Other non-current liabilities
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	0	0
Dollar £m
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	(17,902)	(19,655)
Dollar £m | Cash and cash equivalents
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	33	29
Dollar £m | Financial investments
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	1,125	1,753
Dollar £m | Borrowings
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	(11,068)	(10,729)
Dollar £m | Pre-derivative position
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	(9,910)	(8,947)
Dollar £m | Financing derivatives
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	(7,992)	(10,708)
Dollar £m | Trade and other receivables
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	1,528	1,660
Dollar £m | Trade and other payables
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	(1,793)	(1,795)
Dollar £m | Other non-current liabilities
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	(254)	(315)
Other £m
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	229	270
Other £m | Cash and cash equivalents
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	0	0
Other £m | Financial investments
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	29	66
Other £m | Borrowings
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	(851)	(1,437)
Other £m | Pre-derivative position
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	(822)	(1,371)
Other £m | Financing derivatives
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	1,051	1,641
Other £m | Trade and other receivables
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	0	0
Other £m | Trade and other payables
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	0	0
Other £m | Other non-current liabilities
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	£ 0	£ 0

Financial risk management - Nar

Financial risk management - Narrative (Details)	12 Months Ended
	Mar. 31, 2018GBP (£)	Mar. 31, 2017GBP (£)
Disclosure of detailed information about financial instruments [line items]
Retained cash flow ratio	9.70%	14.90%
Interest cover ratio	4.4	5
Minimum deviation of growth curve to be considered observable for fair value measurement	5.00%
Assets	£ 58,787,000,000	£ 65,840,000,000	[1]
Quadgas HoldCo Limited
Disclosure of detailed information about financial instruments [line items]
Proportion of ownership interest in associate, put/call option equity interest	14.00%	14.00%
Level 3 | Available-for-Sale Investments | Recurring fair value measurement | Enabala Holdings, Inc.
Disclosure of detailed information about financial instruments [line items]
Financial assets, at fair value	£ 5,000,000
Level 3 | Further Acquisition Agreement derivative | Recurring fair value measurement | Quadgas HoldCo Limited
Disclosure of detailed information about financial instruments [line items]
Assets	110,000,000	£ 0
Interest Rate Risk, Fifty Basis Point Decrease In Discount Rate | Sunrun Neptune 2016 LLC
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	6,000,000	8,000,000
Interest Rate Risk, Fifty Basis Point Increase In Discount Rate | Sunrun Neptune 2016 LLC
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	(5,000,000)	£ (8,000,000)
Market Rate Risk, Five Percent Decrease In Offer Price | Quadgas HoldCo Limited
Disclosure of detailed information about financial instruments [line items]
Risk exposure associated with instruments sharing characteristic	£ 39,000,000
[1]	Comparative amounts have been represented to reflect the reclassification of commodity derivative contracts from trade and other receivables and payables, and from other non-current assets and liabilities to derivative financial assets and derivative financial liabilities (see note 16).

Financial risk management - Fai

Financial risk management - Fair value analysis (Details) - GBP (£) £ in Millions	Mar. 31, 2018	Mar. 31, 2017		Mar. 31, 2016
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Assets	£ 4,524	£ 9,891
Liabilities	(1,061)	(2,393)
Net assets	18,848	20,384	[1]
Financial Assets (Liabilities)	3,463	7,498
Level 1
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Assets	2,406	7,717
Liabilities	0	0
Financial Assets (Liabilities)	2,406	7,717
Level 2
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Assets	1,862	2,029
Liabilities	(779)	(1,813)
Financial Assets (Liabilities)	1,083	216
Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Assets	256	145
Liabilities	(282)	(580)
Financial Assets (Liabilities)	(26)	(435)		£ (223)
Investments – available-for-sale
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Assets	2,721	8,037
Investments – available-for-sale | Level 1
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Assets	2,406	7,717
Investments – available-for-sale | Level 2
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Assets	310	315
Investments – available-for-sale | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Assets	5	5
Investments – joint ventures and associates
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Assets	79	41
Investments – joint ventures and associates | Level 1
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Assets	0	0
Investments – joint ventures and associates | Level 2
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Assets	0	0
Investments – joint ventures and associates | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Assets	79	41
Financing derivatives
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Assets	1,545	1,707
Financing derivatives | Level 1
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Assets	0	0
Financing derivatives | Level 2
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Assets	1,544	1,692
Financing derivatives | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Assets	1	15
Commodity contract derivatives
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Assets	69	106
Commodity contract derivatives | Level 1
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Assets	0	0
Commodity contract derivatives | Level 2
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Assets	8	22
Commodity contract derivatives | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Assets	61	84
Further Acquisition Agreement derivative
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Assets	110	0
Further Acquisition Agreement derivative | Level 1
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Assets	0	0
Further Acquisition Agreement derivative | Level 2
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Assets	0	0
Further Acquisition Agreement derivative | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Assets	110	0
Financing derivatives
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Liabilities	(945)	(2,223)
Financing derivatives | Level 1
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Liabilities	0	0
Financing derivatives | Level 2
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Liabilities	(725)	(1,743)
Financing derivatives | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Liabilities	(220)	(480)
Commodity contract derivatives
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Liabilities	(116)	(170)
Commodity contract derivatives | Level 1
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Liabilities	0	0
Commodity contract derivatives | Level 2
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Liabilities	(54)	(70)
Commodity contract derivatives | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Liabilities	£ (62)	£ (100)
[1]	Comparative amounts have been represented to reflect the reclassification of commodity derivative contracts from trade and other receivables and payables, and from other non-current assets and liabilities to derivative financial assets and derivative financial liabilities (see note 16).

Financial risk management - Cha

Financial risk management - Changes in level 3 derivative instruments (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Changes In Fair Value Measurement, Assets (Liabilities) [Roll Forward]
Beginning balance	£ 7,498
Ending balance	3,463	£ 7,498
Level 3
Changes In Fair Value Measurement, Assets (Liabilities) [Roll Forward]
Beginning balance	(435)	(223)
Net gains/(losses) for the year	133	(37)
Purchases	68	61
Settlements	208	(2)
Reclassification into level 3	0	(234)
Ending balance	(26)	(435)
Level 3 | Financing derivatives/ Further Acquisition Agreement derivative
Changes In Fair Value Measurement, Assets (Liabilities) [Roll Forward]
Beginning balance	(465)	(196)
Net gains/(losses) for the year	125	(35)
Purchases	0	0
Settlements	231	0
Reclassification into level 3	0	(234)
Ending balance	(109)	(465)
Gains (losses) on assets held at end of year	125	(35)
Level 3 | Commodity contract derivatives
Changes In Fair Value Measurement, Assets (Liabilities) [Roll Forward]
Beginning balance	(16)	(27)
Net gains/(losses) for the year	8	(2)
Purchases	27	15
Settlements	(20)	(2)
Reclassification into level 3	0	0
Ending balance	(1)	(16)
Gains (losses) on assets held at end of year	35	(21)
Level 3 | Investments in associates and available-for-sale investments
Changes In Fair Value Measurement, Assets (Liabilities) [Roll Forward]
Beginning balance	46	0
Net gains/(losses) for the year	0	0
Purchases	41	46
Settlements	(3)	0
Reclassification into level 3	0	0
Ending balance	£ 84	£ 46

Financial risk management - Imp

Financial risk management - Impacts of reasonably possibly changes in significant level 3 assumptions (Details) - GBP (£) £ in Millions	Mar. 31, 2018	Mar. 31, 2017
Volume forecast
Disclosure of detailed information about financial instruments [line items]
Threshold percentage change for risk exposure with instruments sharing characteristic	10.00%
Commodity contract derivatives | 10% increase in commodity prices
Disclosure of detailed information about financial instruments [line items]
10% increase in commodity prices	£ (1)	£ 1
Commodity contract derivatives | 10% decrease in commodity prices
Disclosure of detailed information about financial instruments [line items]
10% decrease in commodity prices	3	0
Commodity contract derivatives | Volume forecast uplift
Disclosure of detailed information about financial instruments [line items]
Volume forecast uplift	0	(1)
Commodity contract derivatives | Volume forecast reduction
Disclosure of detailed information about financial instruments [line items]
Volume forecast reduction	0	1
Commodity contract derivatives | Plus 10% market area price change
Disclosure of detailed information about financial instruments [line items]
10% market area price change	(6)	(13)
Commodity contract derivatives | Minus 10% market area price change
Disclosure of detailed information about financial instruments [line items]
–10% market area price change	5	9
Financing derivatives | Plus 20 basis points change in Limited Price Inflation (LPI) market curve
Disclosure of detailed information about financial instruments [line items]
20 basis points change in Limited Price Inflation (LPI) market curve	(84)	(93)
Financing derivatives | Minus 20 basis points change in LPI market curve
Disclosure of detailed information about financial instruments [line items]
–20 basis points change in LPI market curve	£ 82	£ 88

Borrowing facilities (Details)

Borrowing facilities (Details) - GBP (£) £ in Millions	Mar. 31, 2018	Mar. 31, 2017
Disclosure of detailed information about borrowings [line items]
Undrawn borrowing facilities	£ 5,683	£ 5,276
Less than 1 year
Disclosure of detailed information about borrowings [line items]
Undrawn borrowing facilities	0	0
In 1 to 2 years
Disclosure of detailed information about borrowings [line items]
Undrawn borrowing facilities	3,910	1,115
In 2 to 3 years
Disclosure of detailed information about borrowings [line items]
Undrawn borrowing facilities	0	2,388
In 3 to 4 years
Disclosure of detailed information about borrowings [line items]
Undrawn borrowing facilities	0	0
In 4 to 5 years
Disclosure of detailed information about borrowings [line items]
Undrawn borrowing facilities	1,773	1,773
More than 5 years
Disclosure of detailed information about borrowings [line items]
Undrawn borrowing facilities	£ 0	£ 0

Borrowing facilities Narrative

Borrowing facilities Narrative (Details) - GBP (£) £ in Millions	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Disclosure of detailed information about borrowings [line items]
Undrawn borrowing facilities	£ 5,683	£ 5,276
Borrowings	26,625	28,638	£ 28,344
Bilateral Committed Credit Facilities
Disclosure of detailed information about borrowings [line items]
Line of credit facility, maximum borrowing capacity	5,438	4,938
Credit Facilities From Syndicates Of Banks
Disclosure of detailed information about borrowings [line items]
Line of credit facility, maximum borrowing capacity	245	338
Facilities Available As Backup To Specific US Borrowings
Disclosure of detailed information about borrowings [line items]
Undrawn borrowing facilities	245	338
Facilities Held As Backup To Commercial Paper And Similar Borrowings
Disclosure of detailed information about borrowings [line items]
Undrawn borrowing facilities	5,438	4,938
Export Credit Agreements
Disclosure of detailed information about borrowings [line items]
Undrawn borrowing facilities	704	562
Borrowings	797	600
2018 Export Credit Agreements
Disclosure of detailed information about borrowings [line items]
Undrawn borrowing facilities	239
Borrowings	261
In 1 to 2 years
Disclosure of detailed information about borrowings [line items]
Undrawn borrowing facilities	3,910	1,115
Borrowings	1,694	1,941
Renegotiated undrawn borrowing facilities	1,255
More than 5 years
Disclosure of detailed information about borrowings [line items]
Undrawn borrowing facilities	0	£ 0
Renegotiated undrawn borrowing facilities	£ 1,380

Subsidiary undertakings, joi185

Subsidiary undertakings, joint ventures and associates - Subsidiaries (Details)	12 Months Ended
	Mar. 31, 2018
Warwick Technology Park Management Company (No 2) Limited
Disclosure of subsidiaries [line items]
Proportion of ownership interest in subsidiary	60.56%
New England Hydro Finance Company, Inc.
Disclosure of subsidiaries [line items]
Proportion of ownership interest in subsidiary	53.704%
New England Hydro-Transmission Corporation
Disclosure of subsidiaries [line items]
Proportion of ownership interest in subsidiary	53.704%
New England Hydro-Transmission Electric Company Inc.
Disclosure of subsidiaries [line items]
Proportion of ownership interest in subsidiary	53.704%
Vermont Green Line Devco, LLC
Disclosure of subsidiaries [line items]
Proportion of ownership interest in subsidiary	90.00%

Subsidiary undertakings, joi186

Subsidiary undertakings, joint ventures and associates - Joint ventures (Details)	Mar. 31, 2018	Mar. 31, 2018
Disclosure of joint ventures [line items]
Proportion of ownership interest in joint venture		50.00%
BritNed Development Limited
Disclosure of joint ventures [line items]
Proportion of ownership interest in joint venture	50.00%	50.00%
Joint Radio Company Limited
Disclosure of joint ventures [line items]
Proportion of ownership interest in joint venture		50.00%
Nemo Link Limited
Disclosure of joint ventures [line items]
Proportion of ownership interest in joint venture		50.00%
NGET/SPT Upgrades Limited
Disclosure of joint ventures [line items]
Proportion of ownership interest in joint venture		50.00%
St William Homes LLP
Disclosure of joint ventures [line items]
Proportion of ownership interest in joint venture		50.00%
Clean Energy Generation, LLC
Disclosure of joint ventures [line items]
Proportion of ownership interest in joint venture		50.00%
Island Park Energy Center, LLC
Disclosure of joint ventures [line items]
Proportion of ownership interest in joint venture		50.00%
Islander East Pipeline Company, LLC
Disclosure of joint ventures [line items]
Proportion of ownership interest in joint venture		50.00%
LI Energy Storage System, LLC
Disclosure of joint ventures [line items]
Proportion of ownership interest in joint venture		50.00%
Swan Lake North Holdings LLC
Disclosure of joint ventures [line items]
Proportion of ownership interest in joint venture		50.00%
IFA2 SAS
Disclosure of joint ventures [line items]
Proportion of ownership interest in joint venture		50.00%

Subsidiary undertakings, joi187

Subsidiary undertakings, joint ventures and associates - Associates (Details)	Mar. 31, 2017	Mar. 31, 2018
Quadgas HoldCo Limited
Disclosure of associates [line items]
Proportion of ownership interest in associate	39.00%	39.00%
Algonquin Gas Transmission, LLC
Disclosure of associates [line items]
Proportion of ownership interest in associate		20.00%
Clean Line Energy Partners LLC
Disclosure of associates [line items]
Proportion of ownership interest in associate		32.00%
Connecticut Yankee Atomic Power Company
Disclosure of associates [line items]
Proportion of ownership interest in associate		19.50%
Direct Global Power, Inc.
Disclosure of associates [line items]
Proportion of ownership interest in associate		26.00%
Energy Impact Fund LP
Disclosure of associates [line items]
Proportion of ownership interest in associate		9.70%
KHB Venture LLC
Disclosure of associates [line items]
Proportion of ownership interest in associate		33.00%
Maine Yankee Atomic Power Company
Disclosure of associates [line items]
Proportion of ownership interest in associate		24.00%
Millennium Pipeline Company, LLC
Disclosure of associates [line items]
Proportion of ownership interest in associate		26.25%
New York Transco LLC
Disclosure of associates [line items]
Proportion of ownership interest in associate		28.30%
Nysearch RMLD, LLC
Disclosure of associates [line items]
Proportion of ownership interest in associate		22.63%
Yankee Atomic Electric Company
Disclosure of associates [line items]
Proportion of ownership interest in associate		34.50%
Coreso SA
Disclosure of associates [line items]
Proportion of ownership interest in associate		15.84%
Energis plc
Disclosure of associates [line items]
Proportion of ownership interest in associate		33.06%

Sensitivites (Details)

Sensitivites (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
UK Pensions | Actuarial assumption of discount rates
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Increase (decrease) in defined benefit plan expense due to reasonably possible increase (decrease) in actuarial assumption	£ 8	£ 9
Increase (decrease) in defined benefit obligation due to reasonably possible increase (decrease) in actuarial assumption BS	£ 1,075	1,305
Change in actuarial assumption, percent	0.50%
UK Pensions | Actuarial assumption of expected rates of inflation
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Increase (decrease) in defined benefit plan expense due to reasonably possible increase (decrease) in actuarial assumption	£ 5	8
Increase (decrease) in defined benefit obligation due to reasonably possible increase (decrease) in actuarial assumption BS	£ 965	1,114
Change in actuarial assumption, percent	0.50%
UK Pensions | Actuarial assumption of expected rates of salary increases
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Increase (decrease) in defined benefit obligation due to reasonably possible increase in actuarial assumption PL	£ 0	2
Increase (decrease) in defined benefit obligation due to reasonably possible increase in actuarial assumption	£ 61	80
Increase in actuarial assumption, percent	0.50%
UK Pensions | Actuarial assumption of life expectancy after retirement
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Increase (decrease) in defined benefit plan expense due to reasonably possible increase (decrease) in actuarial assumption	£ 2	2
Increase (decrease) in defined benefit obligation due to reasonably possible increase (decrease) in actuarial assumption BS	£ 588	673
Increase in actuarial assumption, years	1 year
US Pensions | Actuarial assumption of discount rates
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Increase (decrease) in defined benefit plan expense due to reasonably possible increase (decrease) in actuarial assumption	£ 15	17
Increase (decrease) in defined benefit obligation due to reasonably possible increase (decrease) in actuarial assumption BS	£ 623	669
Change in actuarial assumption, percent	0.50%
US Pensions | Actuarial assumption of expected rates of salary increases
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Increase (decrease) in defined benefit obligation due to reasonably possible increase in actuarial assumption PL	£ 3	3
Increase (decrease) in defined benefit obligation due to reasonably possible increase in actuarial assumption	£ 44	51
Increase in actuarial assumption, percent	0.50%
US Pensions | Actuarial assumption of life expectancy after retirement
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Increase (decrease) in defined benefit plan expense due to reasonably possible increase (decrease) in actuarial assumption	£ 4	4
Increase (decrease) in defined benefit obligation due to reasonably possible increase (decrease) in actuarial assumption BS	£ 359	365
Increase in actuarial assumption, years	1 year
US Pensions | US Healthcare Cost
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Increase (decrease) in defined benefit plan expense due to reasonably possible increase (decrease) in actuarial assumption	£ 31	37
Increase (decrease) in defined benefit obligation due to reasonably possible increase (decrease) in actuarial assumption BS	£ 448	510
Change in actuarial assumption, percent	1.00%
Commodity price risk
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Increase in risk variable, impact on earnings	£ 23	28
Increase in risk variable, impact on other equity reserves	£ 23	28
Change in risk variable, percent	10.00%
Decrease in risk variable, impact on earnings	£ (23)	(29)
Decrease in risk variable, impact on other equity reserves	(23)	(29)
Inflation, UK RPI change
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Change in risk variable, impact on earnings	30	28
Change in risk variable, impact on other equity reserves	£ 0	0
Change in risk variable, percent	0.50%
Currency risk
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Change in risk variable, impact on earnings	£ 48	46
Change in risk variable, impact on other equity reserves	479	604
Change in risk variable, impact on net assets	£ 1,040	988
Change in risk variable, percent	10.00%
Country of domicile | Interest rate risk
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Change in risk variable, impact on earnings	£ 43	64
Change in risk variable, impact on other equity reserves	£ 26	35
Change in risk variable, percent	0.50%
Foreign countries | Interest rate risk
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Change in risk variable, impact on earnings	£ 39	61
Change in risk variable, impact on other equity reserves	£ 17	22
Change in risk variable, percent	0.50%
Fair Value Change | Derivative Financial Instruments
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Change in risk variable, impact on earnings	£ 60	(52)
Change in risk variable, impact on other equity reserves	£ 60	(52)
Change in risk variable, percent	10.00%
Fair Value Change | Commodity Contract Derivative Liabilities
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Change in risk variable, impact on earnings	£ (5)	(6)
Change in risk variable, impact on other equity reserves	£ (5)	(6)
Change in risk variable, percent	10.00%
Other environment related provision | Actuarial assumption of discount rates
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Change in risk variable, impact on earnings	£ 56	67
Change in risk variable, impact on other equity reserves	£ 56	67
Change in risk variable, percent	0.50%
Other environment related provision | Estimated Future Cash Flows Change
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Change in risk variable, impact on earnings	£ 154	175
Change in risk variable, impact on other equity reserves	£ 154	£ 175
Change in risk variable, percent	10.00%

Additional disclosures in re189

Additional disclosures in respect of guaranteed securities - Narrative (Details) - GBP (£) £ in Millions	Oct. 29, 2007	Mar. 31, 2018	Mar. 31, 2017		Mar. 31, 2016
Disclosure of detailed information about borrowings [line items]
Profit (loss)		£ 3,551	£ 7,794	[1]	£ 2,594	[2]
British Transco Finance Inc. £m
Disclosure of detailed information about borrowings [line items]
Borrowings, interest rate		6.625%
Preferred shares issuance rate	3.90%
National Grid Gas plc
Disclosure of detailed information about borrowings [line items]
Preferred shares issuance rate	3.60%
British Transco Finance Inc. £m
Disclosure of detailed information about borrowings [line items]
Profit (loss)		£ 0	£ 0		£ 0
National Grid Plc
Disclosure of detailed information about borrowings [line items]
Proportion of ownership interest in subsidiary		100.00%
British Transco Finance Inc
Disclosure of detailed information about borrowings [line items]
Proportion of ownership interest in subsidiary		100.00%
Vermont Green Line Devco, LLC
Disclosure of detailed information about borrowings [line items]
Proportion of ownership interest in subsidiary		90.00%
Niagara Mohawk Power
Disclosure of detailed information about borrowings [line items]
Proportion of ownership interest in subsidiary		100.00%
[1]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).
[2]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).

Additional disclosures in re190

Additional disclosures in respect of guaranteed securities - Summary Statements of Comprehensive Income (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017		Mar. 31, 2016
Continuing Operations [Abstract]
Revenue	£ 15,250	£ 15,035	[1]	£ 13,212	[2]
Operating Costs [Abstract]
Depreciation and amortisation	(1,530)	(1,481)		(1,311)
Payroll costs	(1,648)	(1,578)		(1,337)
Purchases of electricity	(1,285)	(1,097)		(1,312)
Purchases of gas	(1,543)	(1,219)		(967)
Rates and property taxes	(1,057)	(1,042)		(899)
Balancing services incentive scheme	(1,012)	(1,120)		(907)
Payments to other UK network owners	(1,043)	(1,008)		(971)
Other costs	(2,639)	(3,282)		(2,283)
Operating expense	(11,757)	(11,827)	[1]	(9,987)	[2]
Operating profit	3,493	3,208	[1]	3,225	[2]
Finance income (cost)	(745)	(1,087)		(955)
Dividends receivable	0	0		0
Share of post-tax results of joint ventures and associates	(40)	63	[1]	59	[2]
Profit before tax	2,708	2,184	[1]	2,329	[2]
Tax	884	(374)	[1]	(427)	[2]
Profit (loss) after tax from discontinued operations	(41)	5,984	[1]	692	[2]
Total profit for the year (continuing and discontinued)	3,551	7,794	[1]	2,594	[2]
Amounts recognised in other comprehensive income from continuing operations	371	578		502
Amounts recognised in other comprehensive income from discontinued operations	0	42		71
Other comprehensive income for the year, net of tax	371	620		573
Total comprehensive income for the year	3,922	8,414		3,167
Comprehensive income, attributable to owners of parent	3,922	8,415		3,164
Comprehensive income, attributable to non-controlling interests	0	(1)		3
National Grid plc
Continuing Operations [Abstract]
Revenue	0	0		0
Operating Costs [Abstract]
Depreciation and amortisation	0	0		0
Payroll costs	0	0		0
Purchases of electricity	0	0		0
Purchases of gas	0	0		0
Rates and property taxes	0	0		0
Balancing services incentive scheme	0	0		0
Payments to other UK network owners	0	0		0
Other costs	0	0		0
Operating expense	0	0		0
Operating profit	0	0		0
Finance income (cost)	889	8,177		701
Dividends receivable	950	0		0
Share of post-tax results of joint ventures and associates	1,672	(401)		1,843
Profit before tax	3,511	7,776		2,544
Tax	40	19		47
Profit (loss) after tax from discontinued operations	0	0		0
Total profit for the year (continuing and discontinued)	3,551	7,795		2,591
Amounts recognised in other comprehensive income from continuing operations		578		502
Amounts recognised in other comprehensive income from discontinued operations		42		71
Other comprehensive income for the year, net of tax	371
Total comprehensive income for the year	3,922	8,415		3,164
Comprehensive income, attributable to owners of parent	3,922	8,415		3,164
Comprehensive income, attributable to non-controlling interests	0	0		0
Niagara Mohawk Power Corporation
Continuing Operations [Abstract]
Revenue	2,416	2,388		2,027
Operating Costs [Abstract]
Depreciation and amortisation	(190)	(193)		(162)
Payroll costs	(318)	(326)		(260)
Purchases of electricity	(537)	(511)		(484)
Purchases of gas	(166)	(140)		(86)
Rates and property taxes	(183)	(188)		(155)
Balancing services incentive scheme	0	0		0
Payments to other UK network owners	0	0		0
Other costs	(397)	(435)		(433)
Operating expense	(1,791)	(1,793)		(1,580)
Operating profit	625	595		447
Finance income (cost)	(100)	(101)		(87)
Dividends receivable	0	0		0
Share of post-tax results of joint ventures and associates	0	0		0
Profit before tax	525	494		360
Tax	321	(181)		(141)
Profit (loss) after tax from discontinued operations	0	0		0
Total profit for the year (continuing and discontinued)	846	313		219
Amounts recognised in other comprehensive income from continuing operations		0		(1)
Amounts recognised in other comprehensive income from discontinued operations		0		0
Other comprehensive income for the year, net of tax	1
Total comprehensive income for the year	847	313		218
Comprehensive income, attributable to owners of parent	847	313		218
Comprehensive income, attributable to non-controlling interests	0	0		0
British Transco Finance Inc.
Continuing Operations [Abstract]
Revenue	0	0		0
Operating Costs [Abstract]
Depreciation and amortisation	0	0		0
Payroll costs	0	0		0
Purchases of electricity	0	0		0
Purchases of gas	0	0		0
Rates and property taxes	0	0		0
Balancing services incentive scheme	0	0		0
Payments to other UK network owners	0	0		0
Other costs	0	0		0
Operating expense	0	0		0
Operating profit	0	0		0
Finance income (cost)	0	0		0
Dividends receivable	0	0		0
Share of post-tax results of joint ventures and associates	0	0		0
Profit before tax	0	0		0
Tax	0	0		0
Profit (loss) after tax from discontinued operations	0	0		0
Total profit for the year (continuing and discontinued)	0	0		0
Amounts recognised in other comprehensive income from continuing operations		0		0
Amounts recognised in other comprehensive income from discontinued operations		0		0
Other comprehensive income for the year, net of tax	0
Total comprehensive income for the year	0	0		0
Comprehensive income, attributable to owners of parent	0	0		0
Comprehensive income, attributable to non-controlling interests	0	0		0
National Grid Gas plc
Continuing Operations [Abstract]
Revenue	1,430	1,376		1,244
Operating Costs [Abstract]
Depreciation and amortisation	(245)	(256)		(255)
Payroll costs	(122)	(114)		(115)
Purchases of electricity	0	0		0
Purchases of gas	0	(67)		(75)
Rates and property taxes	(94)	(101)		(101)
Balancing services incentive scheme	0	0		0
Payments to other UK network owners	0	0		0
Other costs	(331)	(394)		(173)
Operating expense	(792)	(932)		(719)
Operating profit	638	444		525
Finance income (cost)	(174)	(253)		(132)
Dividends receivable	0	0		0
Share of post-tax results of joint ventures and associates	8	0		33
Profit before tax	472	191		426
Tax	(103)	16		(56)
Profit (loss) after tax from discontinued operations	17	4,633		735
Total profit for the year (continuing and discontinued)	386	4,840		1,105
Amounts recognised in other comprehensive income from continuing operations		114		8
Amounts recognised in other comprehensive income from discontinued operations		51		(13)
Other comprehensive income for the year, net of tax	272
Total comprehensive income for the year	658	5,005		1,100
Comprehensive income, attributable to owners of parent	658	5,005		1,100
Comprehensive income, attributable to non-controlling interests	0	0		0
Other subsidiaries
Continuing Operations [Abstract]
Revenue	11,495	11,435		10,069
Operating Costs [Abstract]
Depreciation and amortisation	(1,095)	(1,032)		(894)
Payroll costs	(1,208)	(1,138)		(962)
Purchases of electricity	(748)	(586)		(828)
Purchases of gas	(1,377)	(1,012)		(806)
Rates and property taxes	(780)	(753)		(643)
Balancing services incentive scheme	(1,012)	(1,120)		(907)
Payments to other UK network owners	(1,043)	(1,008)		(971)
Other costs	(2,002)	(2,617)		(1,805)
Operating expense	(9,265)	(9,266)		(7,816)
Operating profit	2,230	2,169		2,253
Finance income (cost)	(1,360)	(8,910)		(1,437)
Dividends receivable	0	8,100		620
Share of post-tax results of joint ventures and associates	(40)	63		59
Profit before tax	830	1,422		1,495
Tax	626	(228)		(277)
Profit (loss) after tax from discontinued operations	(58)	1,351		(43)
Total profit for the year (continuing and discontinued)	1,398	2,545		1,175
Amounts recognised in other comprehensive income from continuing operations		177		426
Amounts recognised in other comprehensive income from discontinued operations		(62)		153
Other comprehensive income for the year, net of tax	604
Total comprehensive income for the year	2,002	2,660		1,754
Comprehensive income, attributable to owners of parent	2,002	2,661		1,751
Comprehensive income, attributable to non-controlling interests	0	(1)		3
Consolidation adjustments
Continuing Operations [Abstract]
Revenue	(91)	(164)		(128)
Operating Costs [Abstract]
Depreciation and amortisation	0	0		0
Payroll costs	0	0		0
Purchases of electricity	0	0		0
Purchases of gas	0	0		0
Rates and property taxes	0	0		0
Balancing services incentive scheme	0	0		0
Payments to other UK network owners	0	0		0
Other costs	91	164		128
Operating expense	91	164		128
Operating profit	0	0		0
Finance income (cost)	0	0		0
Dividends receivable	(950)	(8,100)		(620)
Share of post-tax results of joint ventures and associates	(1,680)	401		(1,876)
Profit before tax	(2,630)	(7,699)		(2,496)
Tax	0	0		0
Profit (loss) after tax from discontinued operations	0	0		0
Total profit for the year (continuing and discontinued)	(2,630)	(7,699)		(2,496)
Amounts recognised in other comprehensive income from continuing operations		(291)		(433)
Amounts recognised in other comprehensive income from discontinued operations		11		(140)
Other comprehensive income for the year, net of tax	(877)
Total comprehensive income for the year	(3,507)	(7,979)		(3,069)
Comprehensive income, attributable to owners of parent	(3,507)	(7,979)		(3,069)
Comprehensive income, attributable to non-controlling interests	£ 0	£ 0		£ 0
[1]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).
[2]	Comparatives have been re-presented to reflect the change to a columnar format (see note 1).

Additional disclosures in re191

Additional disclosures in respect of guaranteed securities - Balance Sheet (Details) - GBP (£) £ in Millions	Mar. 31, 2018		Mar. 31, 2017		Mar. 31, 2016	Mar. 31, 2015
Non-current assets
Goodwill	£ 5,444		£ 6,096	[1]
Other intangible assets	899		923	[1]
Property, plant and equipment	39,853		39,825	[1]
Other non-current assets	115		69	[1]
Non-current receivables due from related parties	0		0
Pension assets	1,409		603	[1]
Financial and other investments	3,067		3,183
Derivative financial assets	1,319		1,567	[1]
Total non-current assets	52,106		52,266	[1]
Current assets
Inventories and current intangible assets	341		403	[1]
Trade and other receivables	2,798		2,728	[1]
Current tax assets	114		317	[1]
Amounts owed by subsidiary undertakings	0		0
Financial and other investments	2,694		8,741	[1]	£ 2,998
Derivative financial assets	405		246	[1]
Cash and cash equivalents	329		1,139	[1]	127
Total current assets	6,681		13,574	[1]
Total assets	58,787		65,840	[1]
Current liabilities
Borrowings	(4,447)		(5,496)	[1]
Derivative financial liabilities	(401)		(1,147)	[1]
Trade and other payables	(3,453)		(3,345)	[1]
Amounts owed to subsidiary undertakings	0		0
Current tax liabilities	(123)		(107)	[1]
Provisions	(273)		(416)	[1]
Total current liabilities	(8,697)		(10,511)	[1]
Non-current liabilities
Borrowings	(22,178)		(23,142)	[1]
Derivative financial liabilities	(660)		(1,246)	[1]
Other non-current liabilities	(1,317)		(1,370)	[1]
Amounts owed to subsidiary undertakings	0		0
Deferred tax liabilities	(3,636)		(4,479)	[1]
Pensions and other post-retirement benefit obligations	(1,672)		(2,536)	[1]
Provisions	(1,779)		(2,172)	[1]
Total non-current liabilities	(31,242)		(34,945)	[1]
Total liabilities	(39,939)		(45,456)	[1]
Net assets	18,848		20,384	[1]
Equity
Share capital	452		449	[1]
Share premium account	1,321		1,324	[1]
Retained earnings	21,599		22,582	[1]
Other equity reserves	(4,540)		(3,987)	[1]	(4,523)	£ (4,682)
Total shareholders’ equity	18,832		20,368	[1]
Non-controlling interests	16		16	[1]
Equity	18,848	[2]	20,384	[1]	£ 13,565	£ 11,974
National Grid plc
Non-current assets
Goodwill	0		0
Other intangible assets	0		0
Property, plant and equipment	0		0
Other non-current assets	0		0
Non-current receivables due from related parties	350		342
Pension assets	0		0
Financial and other investments	21,708		17,689
Derivative financial assets	18		149
Total non-current assets	22,076		18,180
Current assets
Inventories and current intangible assets	0		0
Trade and other receivables	1		0
Current tax assets	0		0
Amounts owed by subsidiary undertakings	11,254		12,734
Financial and other investments	938		5,471
Derivative financial assets	308		202
Cash and cash equivalents	0		1,090
Total current assets	12,501		19,497
Total assets	34,577		37,677
Current liabilities
Borrowings	(781)		(1,120)
Derivative financial liabilities	(187)		(533)
Trade and other payables	(62)		(46)
Amounts owed to subsidiary undertakings	(11,809)		(12,012)
Current tax liabilities	0		(3)
Provisions	0		0
Total current liabilities	(12,839)		(13,714)
Non-current liabilities
Borrowings	(773)		(1,262)
Derivative financial liabilities	(41)		(272)
Other non-current liabilities	0		0
Amounts owed to subsidiary undertakings	(2,091)		(2,058)
Deferred tax liabilities	(1)		(3)
Pensions and other post-retirement benefit obligations	0		0
Provisions	0		0
Total non-current liabilities	(2,906)		(3,595)
Total liabilities	(15,745)		(17,309)
Net assets	18,832		20,368
Equity
Share capital	452		449
Share premium account	1,321		1,324
Retained earnings	21,599		22,582
Other equity reserves	(4,540)		(3,987)
Total shareholders’ equity	18,832		20,368
Non-controlling interests	0		0
Equity	18,832		20,368
Niagara Mohawk Power Corporation
Non-current assets
Goodwill	691		763
Other intangible assets	3		0
Property, plant and equipment	6,148		6,553
Other non-current assets	3		5
Non-current receivables due from related parties	0		0
Pension assets	231		223
Financial and other investments	30		31
Derivative financial assets	2		2
Total non-current assets	7,108		7,577
Current assets
Inventories and current intangible assets	36		38
Trade and other receivables	515		494
Current tax assets	0		0
Amounts owed by subsidiary undertakings	130		505
Financial and other investments	15		29
Derivative financial assets	7		13
Cash and cash equivalents	4		4
Total current assets	707		1,083
Total assets	7,815		8,660
Current liabilities
Borrowings	(51)		(55)
Derivative financial liabilities	(36)		(42)
Trade and other payables	(318)		(311)
Amounts owed to subsidiary undertakings	0		0
Current tax liabilities	(202)		(156)
Provisions	(23)		0
Total current liabilities	(630)		(564)
Non-current liabilities
Borrowings	(2,087)		(2,345)
Derivative financial liabilities	(18)		(26)
Other non-current liabilities	(281)		(324)
Amounts owed to subsidiary undertakings	0		0
Deferred tax liabilities	(626)		(1,178)
Pensions and other post-retirement benefit obligations	(765)		(889)
Provisions	(248)		(298)
Total non-current liabilities	(4,025)		(5,060)
Total liabilities	(4,655)		(5,624)
Net assets	3,160		3,036
Equity
Share capital	133		149
Share premium account	2,194		2,431
Retained earnings	830		456
Other equity reserves	3		0
Total shareholders’ equity	3,160		3,036
Non-controlling interests	0		0
Equity	3,160		3,036
British Transco Finance Inc.
Non-current assets
Goodwill	0		0
Other intangible assets	0		0
Property, plant and equipment	0		0
Other non-current assets	0		0
Non-current receivables due from related parties	0		239
Pension assets	0		0
Financial and other investments	0		0
Derivative financial assets	0		0
Total non-current assets	0		239
Current assets
Inventories and current intangible assets	0		0
Trade and other receivables	0		0
Current tax assets	0		0
Amounts owed by subsidiary undertakings	220		6
Financial and other investments	0		0
Derivative financial assets	0		0
Cash and cash equivalents	0		0
Total current assets	220		6
Total assets	220		245
Current liabilities
Borrowings	(218)		(5)
Derivative financial liabilities	0		0
Trade and other payables	0		0
Amounts owed to subsidiary undertakings	0		0
Current tax liabilities	0		0
Provisions	0		0
Total current liabilities	(218)		(5)
Non-current liabilities
Borrowings	0		(239)
Derivative financial liabilities	0		0
Other non-current liabilities	0		0
Amounts owed to subsidiary undertakings	0		0
Deferred tax liabilities	0		0
Pensions and other post-retirement benefit obligations	0		0
Provisions	0		0
Total non-current liabilities	0		(239)
Total liabilities	(218)		(244)
Net assets	2		1
Equity
Share capital	0		0
Share premium account	0		0
Retained earnings	2		1
Other equity reserves	0		0
Total shareholders’ equity	2		1
Non-controlling interests	0		0
Equity	2		1
National Grid Gas plc
Non-current assets
Goodwill	0		0
Other intangible assets	107		125
Property, plant and equipment	4,433		4,358
Other non-current assets	94		9
Non-current receivables due from related parties	3,426		3,426
Pension assets	412		45
Financial and other investments	101		95
Derivative financial assets	591		813
Total non-current assets	9,164		8,871
Current assets
Inventories and current intangible assets	22		20
Trade and other receivables	276		360
Current tax assets	0		0
Amounts owed by subsidiary undertakings	708		1,965
Financial and other investments	863		1,835
Derivative financial assets	46		51
Cash and cash equivalents	271		(9)
Total current assets	2,186		4,222
Total assets	11,350		13,093
Current liabilities
Borrowings	(675)		(833)
Derivative financial liabilities	(68)		(185)
Trade and other payables	(347)		(342)
Amounts owed to subsidiary undertakings	(624)		(2,151)
Current tax liabilities	(26)		(9)
Provisions	(66)		(184)
Total current liabilities	(1,806)		(3,704)
Non-current liabilities
Borrowings	(3,635)		(3,879)
Derivative financial liabilities	(157)		(234)
Other non-current liabilities	(181)		(204)
Amounts owed to subsidiary undertakings	(500)		(756)
Deferred tax liabilities	(441)		(369)
Pensions and other post-retirement benefit obligations	0		0
Provisions	(129)		(104)
Total non-current liabilities	(5,043)		(5,546)
Total liabilities	(6,849)		(9,250)
Net assets	4,501		3,843
Equity
Share capital	45		45
Share premium account	204		204
Retained earnings	2,929		2,268
Other equity reserves	1,323		1,326
Total shareholders’ equity	4,501		3,843
Non-controlling interests	0		0
Equity	4,501		3,843
Other subsidiaries
Non-current assets
Goodwill	4,753		5,333
Other intangible assets	789		798
Property, plant and equipment	29,272		28,914
Other non-current assets	18		55
Non-current receivables due from related parties	2,593		2,576
Pension assets	766		335
Financial and other investments	12,340		12,429
Derivative financial assets	708		603
Total non-current assets	51,239		51,043
Current assets
Inventories and current intangible assets	283		345
Trade and other receivables	2,006		1,874
Current tax assets	307		317
Amounts owed by subsidiary undertakings	11,253		12,083
Financial and other investments	878		1,406
Derivative financial assets	0		174
Cash and cash equivalents	54		54
Total current assets	14,781		16,253
Total assets	66,020		67,296
Current liabilities
Borrowings	(2,722)		(3,483)
Derivative financial liabilities	(66)		(581)
Trade and other payables	(2,726)		(2,646)
Amounts owed to subsidiary undertakings	(11,132)		(13,130)
Current tax liabilities	(88)		61
Provisions	(184)		(232)
Total current liabilities	(16,918)		(20,011)
Non-current liabilities
Borrowings	(15,683)		(15,417)
Derivative financial liabilities	(444)		(714)
Other non-current liabilities	(855)		(842)
Amounts owed to subsidiary undertakings	(3,778)		(3,769)
Deferred tax liabilities	(2,568)		(2,929)
Pensions and other post-retirement benefit obligations	(907)		(1,647)
Provisions	(1,402)		(1,770)
Total non-current liabilities	(25,637)		(27,088)
Total liabilities	(42,555)		(47,099)
Net assets	23,465		20,197
Equity
Share capital	182		182
Share premium account	9,032		8,033
Retained earnings	14,217		11,914
Other equity reserves	18		52
Total shareholders’ equity	23,449		20,181
Non-controlling interests	16		16
Equity	23,465		20,197
Consolidation adjustments
Non-current assets
Goodwill	0		0
Other intangible assets	0		0
Property, plant and equipment	0		0
Other non-current assets	0		0
Non-current receivables due from related parties	(6,369)		(6,583)
Pension assets	0		0
Financial and other investments	(31,112)		(27,061)
Derivative financial assets	0		0
Total non-current assets	(37,481)		(33,644)
Current assets
Inventories and current intangible assets	0		0
Trade and other receivables	0		0
Current tax assets	(193)		0
Amounts owed by subsidiary undertakings	(23,565)		(27,293)
Financial and other investments	0		0
Derivative financial assets	44		(194)
Cash and cash equivalents	0		0
Total current assets	(23,714)		(27,487)
Total assets	(61,195)		(61,131)
Current liabilities
Borrowings	0		0
Derivative financial liabilities	(44)		194
Trade and other payables	0		0
Amounts owed to subsidiary undertakings	23,565		27,293
Current tax liabilities	193		0
Provisions	0		0
Total current liabilities	23,714		27,487
Non-current liabilities
Borrowings	0		0
Derivative financial liabilities	0		0
Other non-current liabilities	0		0
Amounts owed to subsidiary undertakings	6,369		6,583
Deferred tax liabilities	0		0
Pensions and other post-retirement benefit obligations	0		0
Provisions	0		0
Total non-current liabilities	6,369		6,583
Total liabilities	30,083		34,070
Net assets	(31,112)		(27,061)
Equity
Share capital	(360)		(376)
Share premium account	(11,430)		(10,668)
Retained earnings	(17,978)		(14,639)
Other equity reserves	(1,344)		(1,378)
Total shareholders’ equity	(31,112)		(27,061)
Non-controlling interests	0		0
Equity	£ (31,112)		£ (27,061)
[1]	Comparative amounts have been represented to reflect the reclassification of commodity derivative contracts from trade and other receivables and payables, and from other non-current assets and liabilities to derivative financial assets and derivative financial liabilities (see note 16).
[2]	Refer to note 7 for the effect of the share consolidation and the special dividend.

Additional disclosures in re192

Additional disclosures in respect of guaranteed securities - Cash Flow (Details) - GBP (£) £ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Disclosure of information about consolidated structured entities [line items]
Net cash flow from operating activities – continuing operations	£ 4,710	£ 4,320	£ 4,292
Net cash (used in)/inflow from operating activities – discontinued operations	(207)	909	1,076
Net cash flow from/(used in) investing activities – continuing operations	2,237	(3,634)	(3,459)
Net cash flow used in investing activities – discontinued operations	0	(680)	(577)
Net cash flow (used in)/from financing activities – continuing operations	(7,316)	(1,542)	(1,205)
Net cash flow (used in)/from financing activities – discontinued operations	(231)	1,611	(123)
Net (decrease)/increase in cash and cash equivalents	(807)	984	4
National Grid plc
Disclosure of information about consolidated structured entities [line items]
Net cash flow from operating activities – continuing operations	35	53	57
Net cash (used in)/inflow from operating activities – discontinued operations	0	0	0
Net cash flow from/(used in) investing activities – continuing operations	4,660	4,181	502
Net cash flow used in investing activities – discontinued operations	0	0	0
Net cash flow (used in)/from financing activities – continuing operations	(5,785)	(3,146)	(555)
Net cash flow (used in)/from financing activities – discontinued operations	0	0	0
Net (decrease)/increase in cash and cash equivalents	(1,090)	1,088	4
Niagara Mohawk Power Corporation
Disclosure of information about consolidated structured entities [line items]
Net cash flow from operating activities – continuing operations	662	757	580
Net cash (used in)/inflow from operating activities – discontinued operations	0	0	0
Net cash flow from/(used in) investing activities – continuing operations	(473)	(469)	(440)
Net cash flow used in investing activities – discontinued operations	0	0	0
Net cash flow (used in)/from financing activities – continuing operations	(189)	(288)	(148)
Net cash flow (used in)/from financing activities – discontinued operations	0	0	0
Net (decrease)/increase in cash and cash equivalents	0	0	(8)
British Transco Finance Inc.
Disclosure of information about consolidated structured entities [line items]
Net cash flow from operating activities – continuing operations	0	0	0
Net cash (used in)/inflow from operating activities – discontinued operations	0	0	0
Net cash flow from/(used in) investing activities – continuing operations	15	15	13
Net cash flow used in investing activities – discontinued operations	0	0	0
Net cash flow (used in)/from financing activities – continuing operations	(15)	(15)	(13)
Net cash flow (used in)/from financing activities – discontinued operations	0	0	0
Net (decrease)/increase in cash and cash equivalents	0	0	0
National Grid Gas plc
Disclosure of information about consolidated structured entities [line items]
Net cash flow from operating activities – continuing operations	888	918	599
Net cash (used in)/inflow from operating activities – discontinued operations	(98)	450	1,144
Net cash flow from/(used in) investing activities – continuing operations	656	215	56
Net cash flow used in investing activities – discontinued operations	0	5,618	(562)
Net cash flow (used in)/from financing activities – continuing operations	(1,041)	(8,322)	(1,185)
Net cash flow (used in)/from financing activities – discontinued operations	(125)	1,120	(63)
Net (decrease)/increase in cash and cash equivalents	280	(1)	(11)
Other subsidiaries
Disclosure of information about consolidated structured entities [line items]
Net cash flow from operating activities – continuing operations	3,125	2,592	3,056
Net cash (used in)/inflow from operating activities – discontinued operations	(109)	459	(68)
Net cash flow from/(used in) investing activities – continuing operations	(1,759)	(1,118)	(1,721)
Net cash flow used in investing activities – discontinued operations	0	(6,298)	(15)
Net cash flow (used in)/from financing activities – continuing operations	(1,148)	3,771	(1,173)
Net cash flow (used in)/from financing activities – discontinued operations	(106)	491	(60)
Net (decrease)/increase in cash and cash equivalents	3	(103)	19
Consolidation adjustments
Disclosure of information about consolidated structured entities [line items]
Net cash flow from operating activities – continuing operations	0	0	0
Net cash (used in)/inflow from operating activities – discontinued operations	0	0	0
Net cash flow from/(used in) investing activities – continuing operations	(862)	(6,458)	(1,869)
Net cash flow used in investing activities – discontinued operations	0	0	0
Net cash flow (used in)/from financing activities – continuing operations	862	6,458	1,869
Net cash flow (used in)/from financing activities – discontinued operations	0	0	0
Net (decrease)/increase in cash and cash equivalents	0	0	0
Subsidiary Undertaking
Disclosure of information about consolidated structured entities [line items]
Dividends received	£ 950	£ 6,006	£ 930

Additional disclosures in re193

Additional disclosures in respect of guaranteed securities - Parent Company Borrowings (Details) - GBP (£) £ in Millions	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Disclosure of detailed information about borrowings [line items]
Borrowings	£ 26,625	£ 28,638	£ 28,344
Less than 1 year
Disclosure of detailed information about borrowings [line items]
Borrowings	4,447	5,496
In 1 to 2 years
Disclosure of detailed information about borrowings [line items]
Borrowings	1,694	1,941
In 2 to 3 years
Disclosure of detailed information about borrowings [line items]
Borrowings	2,347	1,821
In 3 to 4 years
Disclosure of detailed information about borrowings [line items]
Borrowings	1,873	2,453
In 4 to 5 years
Disclosure of detailed information about borrowings [line items]
Borrowings	1,469	1,921
National Grid plc
Disclosure of detailed information about borrowings [line items]
Borrowings	1,554	2,382
National Grid plc | Less than 1 year
Disclosure of detailed information about borrowings [line items]
Borrowings	781	1,120
National Grid plc | In 1 to 2 years
Disclosure of detailed information about borrowings [line items]
Borrowings	438	515
National Grid plc | In 2 to 3 years
Disclosure of detailed information about borrowings [line items]
Borrowings	0	425
National Grid plc | In 3 to 4 years
Disclosure of detailed information about borrowings [line items]
Borrowings	335	0
National Grid plc | In 4 to 5 years
Disclosure of detailed information about borrowings [line items]
Borrowings	0	322
National Grid plc | More than 5 years
Disclosure of detailed information about borrowings [line items]
Borrowings	£ 0	£ 0

Post balance sheet events Narra

Post balance sheet events Narrative (Details)	12 Months Ended
	Mar. 31, 2018
Quadgas HoldCo Limited
Disclosure of associates [line items]
Proportion of ownership interest in associate, excluding put/call option equity interest	25.00%