EXHIBIT 10.27

Private & Confidential



                              DATED 24 APRIL 1998
                              -------------------

                             BERKSHIRE FUND IV, LP               (1) 
                            BERKSHIRE INVESTORS LLC              (2)   
                            BERKSHIRE PARTNERS LLC               (3)   
                           CANDOVER INVESTMENTS PLC              (4)   
                          CANDOVER (TRUSTEES) LIMITED            (5)   
                           CANDOVER PARTNERS LIMITED             (6)   
                           CANDOVER PARTNERS LIMITED             (7)   
                           CANDOVER PARTNERS LIMITED             (8)   
                           CANDOVER PARTNERS LIMITED             (9)   
                       CROWN CASTLE INTERNATIONAL CORP.         (10)   
                  TELEDIFFUSION DE FRANCE INTERNATIONAL S.A.    (11)   
                       DIGITAL FUTURE INVESTMENTS B.V.          (12)   
               CASTLE TRANSMISSION SERVICES (HOLDINGS) LIMITED  (13)    

                       _________________________________

                                   AGREEMENT
      FOR THE SALE AND PURCHASE OF CERATIN SHARES OF CASTLE TRANSMISSION
 SERVICES (HOLDINGS) LIMITED, FOR THE AMENDMENT OF THE SHAREHOLDERS' AGREEMENT
         IN RESPECT OF CASTLE TRANSMISSION SERVICES (HOLDINGS) LIMITED
                    AND FOR THE GRANTING OF CERTAIN OPTIONS

                       _________________________________



                                  NORTON ROSE
                                    London

 
                                   CONTENTS
                                   --------



CLAUSE                  HEADING                             PAGE
                                                          
1     Definitions and interpretation.........................  2
2     Conditions precedent...................................  3
3     Sale of the Sale Shares................................  4
4     Completion of the sale of the Sale Shares..............  5
5     Shareholders' Agreement issues.........................  7
6     Options................................................  8
7     Continuation of Shareholders' Agreement................  8
8     Warranties.............................................  8
9     Confidentiality........................................  8
10    Public Announcements...................................  9
11    Further assurances.....................................  9
12    Modification and assignment............................  9
13    Effect of waiver.......................................  9
14    Entire agreement....................................... 10
15    Governing law and jurisdiction......................... 10
16    Notices................................................ 11
17    Counterparts........................................... 12

SCHEDULE

1     The Sale Shares........................................ 13
2     Deed of Adherence...................................... 14
3     Options................................................ 16
4     The Resolution......................................... 21
5     Standby Option Agreement............................... 22


 
THIS AGREEMENT is dated 24 April 1998 and is made AMONG:

(1)  BERKSHIRE FUND IV, LP, a Massachusetts limited partnership;

(2)  BERKSHIRE INVESTORS LLC, a Massachusetts limited liability corporation;

(3)  BERKSHIRE PARTNERS LLC, a Massachusetts limited liability corporation;

(4)  CANDOVER INVESTMENTS PLC, a company incorporated in England and Wales;

(5)  CANDOVER (TRUSTEES) LIMITED, a company incorporated in England and Wales;

(6)  CANDOVER PARTNERS LIMITED, a company incorporated in England and Wales (as
     general partner of the Candover 1994 UK Limited Partnership);

(7)  CANDOVER PARTNERS LIMITED, a company incorporated in England and Wales (as
     general partner of the Candover 1994 UK No. 2 Limited Partnership);

(8)  CANDOVER PARTNERS LIMITED, a company incorporated in England and Wales (as
     general partner of the Candover 1994 US No. 1 Limited Partnership);

(9)  CANDOVER PARTNERS LIMITED, a company incorporated in England and Wales (as
     general partner of the Candover 1994 US No. 2 Limited Partnership);

(10) CROWN CASTLE INTERNATIONAL CORP. ("CCIC"), a Delaware corporation;

(11) TELEDIFFUSION DE FRANCE INTERNATIONAL S.A. ("TDFI"), a company incorporated
     in France;

(12) DIGITAL FUTURE INVESTMENTS B.V. (the "PURCHASER"), a company incorporated
     in The Netherlands;

(13) CASTLE TRANSMISSION SERVICES (HOLDINGS) LIMITED (the "COMPANY"), a company
     incorporated in England and Wales.

WHEREAS

(A)  The parties to this Agreement (other than the Purchaser, the Company and
     Berkshire Partners LLC) are shareholders of the Company.

(B)  The parties to this Agreement (other than the Purchaser) are parties to a
     shareholders agreement in respect of the Company dated 23 January 1997 (as
     amended by a Deed of Adherence dated 2 May 1997).

 
(C)  The Purchaser has agreed to purchase certain shares of the Company from the
     Berkshire Vendors and the Candover Vendors (each as defined in Clause 1)
     and the parties have agreed to make certain consequential amendments to the
     arrangements between them as shareholders of the Company, in each case on
     the terms set out in this Agreement.

1    DEFINITIONS AND INTERPRETATION
     ------------------------------

1.1  In this Agreement unless the context otherwise requires:

     "BERKSHIRE" has the meaning ascribed to such expression in the
     Shareholders' Agreement;

     "BERKSHIRE SALE SHARES" means the aggregate of the number of Ordinary
     Shares of 1p each of the Company and Redeemable Preference Shares of 1p
     each of the Company respectively set out opposite the name of each
     Berkshire Vendor in columns (2) and (3) of schedule 1;

     "BERKSHIRE VENDORS'" means Berkshire Fund IV, LP and Berkshire Investors
     LLC;

     "CANDOVER" has the meaning ascribed to such expression in the Shareholders'
     Agreement;

     "CANDOVER SALE SHARES" means the aggregate of the number of Ordinary Shares
     of 1p each of the Company and Redeemable Preference Shares of 1p each of
     the Company respectively set out opposite the name of each Candover Vendor
     in columns (2) and (3) of schedule 1;

     "CANDOVER VENDORS" means Candover Investments PLC, Candover (Trustees)
     Limited and Candover Partners Limited;

     "COMPLETION" means the completion of the sale and purchase of the Sale
     Shares by the performance by the parties of their respective obligations
     under clause 4;

     "DEED OF ADHERENCE" means the deed in the form set out in schedule 2;

     "GOVERNANCE AGREEMENT" means an agreement in the agreed form to be entered
     into by CCIC (1) and TdFI (2);

     "OPTION AGREEMENT" means an agreement in the agreed form to be entered into
     by the Purchaser (1), CCIC (2) and TdFI (3);

     "SALE SHARES" means the Berkshire Sale Shares and the Candover Sale Shares;

                                       2

 
     "SHARE EXCHANGE AGREEMENT" means an agreement to be entered into on the
     date hereof by the Company (1) CCIC (2) TDFI (3) and others (4) providing
     for the exchange of all shares held in the capital of the Company (other
     than those held at the date hereof by CCIC and TdFI (but not the
     Purchaser)) for fully paid shares of common stock (or, in the case of the
     Purchaser, fully paid shares of class A common stock) of CCIC for the same
     consideration per share;

     "SHAREHOLDERS' AGREEMENT" means the shareholders' agreement in respect of
     the Company dated 23 January 1997 (as amended by a Deed of Adherence dated
     2 May 1997) between the parties to this Agreement (other than the
     Purchaser);

     "STOCKHOLDERS AGREEMENT" means an agreement in the agreed form to be
     entered into by CCIC (1) and others (2).

1.2  In this Agreement unless the context otherwise requires:

     (a)  a document expressed to be "IN THE AGREED FORM" means a document in a
          form which has been agreed by the parties contemporaneously with or
          before the execution of this Agreement and which has, for the purposes
          of identification, been signed or initialled by them or on their
          behalf;

     (b)  references to a clause or schedule are to a clause of, or a schedule
          to, this Agreement, references to this Agreement include its schedules
          and references in a schedule or part of a schedule to a paragraph are
          to a paragraph of that schedule or that part of that schedule;

     (c)  references to this Agreement or any other document or to any specified
          provision of this Agreement or any other document are to this
          Agreement, that document or that provision as in force for the time
          being and as amended from time to time in accordance with the terms of
          this Agreement or that document or, as the case may be, with the
          agreement of the relevant parties;

     (d)  words importing the singular include the plural and vice versa, words
          importing a gender include every gender and references to persons
          include corporations, partnerships and other unincorporated
          associations or bodies of persons;

     (e)  the contents table and the descriptive headings to clauses, schedules
          and paragraphs are inserted for convenience only, have no legal effect
          and shall be ignored in the interpretation of this Agreement.

2    CONDITIONS PRECEDENT
     --------------------

2.1  The obligations of the parties under this Agreement (other than those
     contained in clause 2.3) are conditional on the fulfilment of the
     conditions listed in clause 2.2 on or prior to 

                                       3

 
     the date hereof, and no right under this Agreement (other than any which
     may arise under clause 2.3) shall arise until such conditions shall have
     been fulfilled.

2.2  The conditions referred to in clause 2.1 are:

     (a)  the due execution by all parties thereto of the Share Exchange
          Agreement;

     (b)  the due execution by all parties thereto of the Option Agreement.

2.3  Each of the parties shall use all reasonable endeavours to ensure (so far
     as it is able) that the conditions listed in clause 2.2 shall be fulfilled
     on the date hereof.

3    SALE OF THE SALE SHARES
     -----------------------

3.1  Each of the Berkshire Vendors shall sell to the Purchaser, and the
     Purchaser shall purchase from each of the Berkshire Vendors, the number of
     Berkshire Sale Shares set opposite its name in columns (2) and (3) of
     schedule 1.  Each of the Berkshire Vendors shall sell and transfer that
     number of the Berkshire Sale Shares set opposite its name in columns (2)
     and (3) of schedule 1 free from all encumbrances, claims, liens, charges,
     equities and other rights exercisable by third parties and (subject
     thereto) with full title guarantee, but no other warranties,
     representations, or assurances are given in respect of the Berkshire Sale
     Shares (except to the extent expressly contained in this Agreement).

3.2  The consideration for the sale of the Berkshire Sale Shares shall be the
     payment by the Purchaser on Completion to each of the Berkshire Vendors of
     the aggregate of the sums set out opposite its name in columns (4) and (5)
     of schedule 1.

3.3  Each of the Candover Vendors shall sell to the Purchaser, and the Purchaser
     shall purchase from each of the Candover Vendors, the number of Candover
     Sale Shares set opposite its name in columns (2) and (3) of schedule 1.
     Each of the Candover Vendors shall sell and transfer that number of
     Candover Sale Shares set opposite its name in columns (2) and (3) free from
     all encumbrances, claims, liens, charges, equities and other rights
     exercisable by third parties and (subject thereto) with full title
     guarantee, but no other warranties, representations or assurances are given
     in respect of the Candover Sale Shares (except to the extent expressly
     contained in this Agreement).

3.4  The consideration for the sale of Candover Sale Shares shall be the payment
     by the Purchaser on Completion to each of the Candover Vendors of the
     aggregate of the sums set out opposite its name in columns (4) and (5) of
     schedule 1.

3.5  Each of the parties hereby:

     (a)  waives any rights of pre-emption conferred on it by the Articles of
          Association of the Company or by the Shareholders' Agreement or
          otherwise over the Sale 

                                       4

 
          Shares hereby agreed to be sold by the Berkshire Vendors and the
          Candover Vendors and in respect of any shares which may be sold
          pursuant to schedule 3 or the agreement set out in schedule 5;

     (b)  consents to the sale of the Sale Shares to the Purchaser for the
          purpose of complying with the requirements of the Articles of
          Association of the Company and the Shareholders' Agreement and for the
          same purpose consents to the sale of any shares pursuant to schedule 3
          or the agreement set out in schedule 5;

     (c)  waives any tag along rights which it may have under clause 9.6 of the
          Shareholders' Agreement in respect of the sale of the Sale Shares to
          the Purchaser and in respect of any sale of shares pursuant to
          schedule 3 or the agreement set out in schedule 5;

     (d)  consents to all transactions contemplated by the Share Exchange
          Agreement, the Governance Agreement, the Stockholders Agreement and
          the Option Agreement and waives all rights of pre-emption conferred by
          the Articles of Association of the Company or the Shareholders'
          Agreement in respect of the transactions contemplated by such
          agreements; and

     (e)  agrees to take, or to join in taking, such other action as is within
          its power to effect the transactions contemplated by the Share
          Exchange Agreement, the Governance Agreement, the Stockholders
          Agreement, the Option Agreement and the agreement set out in schedule
          5, including (without limitation) procuring that transfers of shares
          contemplated by those agreements are resolved to be registered
          (subject to their being duly stamped).

3.6  The Purchaser shall not be obliged to complete the purchase of any of the
     Sale Shares unless the purchase of all the Sale Shares is completed
     simultaneously.

4    COMPLETION OF THE SALE OF THE SALE SHARES
     -----------------------------------------

4.1  Subject to the conditions set out in clause 2.2 having been fulfilled or
     waived on the date hereof, Completion shall take place on the date hereof
     and all (but not part only unless the parties shall so agree) of the
     following business shall be transacted:

     (a)  each of the Berkshire Vendors and the Candover Vendors shall deliver
          to the Purchaser transfers in respect of the number of the Sale Shares
          set opposite its name in columns (2) and (3) of schedule 1, duly
          executed and completed in favour of the Purchaser, together with
          certificates therefor and the duly executed powers of attorney or
          other authorities under which the transfers have been executed;

     (b)  the parties shall join in procuring (so far as they are able) that:

                                       5

 
          (i)  the transfers mentioned in clause 4.1(a) shall be resolved to be
               registered (subject only to their being duly stamped);

          (ii) a share certificate is issued to the Purchaser in respect of the
               Sale Shares (subject to the transfers mentioned in clause 4.1(a)
               having been duly stamped) and share certificates are issued to
               the Berkshire Vendors and the Candover Vendors in respect of the
               balance of the shares of the Company respectively owned by them
               following the sale of the Sale Shares;

     (c)  the Purchaser shall execute and deliver the Deed of Adherence;

     (d)  the Purchaser shall pay the consideration for the sale of the
          Berkshire Sale Shares by electronic funds transfer for value on the
          day of Completion to the following account:

          National Westminster Bank, London

          SWIFT Code: NWBKGB2L

          Fleet National Bank Account

          Account Number:  4400204622421

          Reference:  Notify on receipt Ms Leslie Obryon, 617-346-5199

          and payment of such consideration into such account shall constitute a
          good discharge to the Purchaser in respect of it and the Purchaser
          shall have no obligation as to the distribution or allocation of such
          consideration between the Berkshire Vendors;

     (e)  the Purchaser shall pay the consideration for the sale of the Candover
          Sale Shares by electronic funds transfer for value on the day of
          Completion to the following account:

          Bank:               Bank of Scotland     
                              London Chief Office  
                              PO Box 267           
                              38 Threadneedle Street
                              London EC2P 2EH       

          Sort Code:          12 01 03
          SWIFT Code:         BOFSGB2L
          Account Name:       Candover Partners Limited

                                       6

 
          Account No:         00214728

          and payment of such consideration into such account shall constitute a
          good discharge to the Purchaser in respect of it and the Purchaser
          shall have no obligation as to the distribution or allocation of such
          consideration among the Candover Vendors; and

     (f)  Berkshire, Candover, TdFI and CCIC shall join in procuring that a
          special resolution of the Company in the agreed form shall be passed
          to change the articles of association of the Company.

5    SHAREHOLDERS' AGREEMENT ISSUES
     ------------------------------

5.1  The provisions of clauses 5.2 to 5.4 and clause 6 shall have effect only if
     Completion shall have occurred.

5.2  For the purposes of the Shareholders' Agreement, the expression "TdFI"
     shall henceforth be treated as meaning TeleDiffusion de France
     International S.A. and the Purchaser taken together and, for the purposes
     of the Shareholders' Agreement, TeleDiffusion de France International S.A.
     and the Purchaser shall be treated as if they were one and the same person.

5.3  Until the earlier of the date on which the parties to this Agreement agree
     otherwise and the date on which Berkshire shall hold less than 10 per cent.
     of the equity share capital of the Company:

     (a)  for the purposes of ascertaining the rights of Berkshire under the
          Shareholders' Agreement, the parties agree that Berkshire shall at all
          times and for all purposes be treated as if it held 15 per cent. in
          number of the Ordinary Shares of 1p each of the Company in issue or,
          as the case may be, 15 per cent. in value of the aggregate of the
          Ordinary Shares of 1p each of the Company and of the Preference Shares
          of 1p each of the Company in issue or, as the case may be, 15 per
          cent. of the equity share capital of the Company and, in each such
          case, the actual percentage shareholdings of the other shareholders of
          the Company shall (for the purposes of this clause 5.3(a)) be treated
          as if they had been reduced on a pro-rata basis to give effect to the
          foregoing;

     (b)  each of the parties to this Agreement (other than Berkshire and the
          Company) agrees that (for so long as it continues to be a shareholder
          of the Company) it will exercise its voting rights at any meeting of
          the shareholders of the Company in a manner consistent with ensuring
          that any resolution proposed thereat shall not be passed if (a) that
          party has been notified by Berkshire that it proposes to vote against
          such resolution and such resolution would not be passed if Berkshire
          held 15 per cent. in number of the Ordinary Shares of 1p each of the
          Company in issue and were to vote against such resolution or (b) that
          party has been notified by Berkshire and Candover that each of them
          proposes to vote against such resolution.

                                       7

 
5.4  If at any time Candover shall hold less than 15 per cent. in number of the
     Ordinary Shares of 1p each of the Company in issue, the provisions of
     clause 5.3 shall also apply mutatis mutandis for its benefit save that all
     references in clause 5.3 to "Berkshire" shall be replaced by references to
     "Candover".

5.5  CCIC undertakes to TdFI that it will not exercise any of its rights to
     subscribe for shares of the Company under any of the warrants issued to it
     by the Company if the result thereof would be that CCIC would thereby hold
     more than 50 per cent. of the issued share capital of the Company.

6    OPTIONS
     -------

6.1  The provisions of schedule 3 shall have effect for the benefit of Berkshire
     and Candover.

7    CONTINUATION OF SHAREHOLDERS' AGREEMENT
     ---------------------------------------

7.1  Save as amended by this Agreement, the Shareholders' Agreement shall
     continue in full force and effect.

8    WARRANTIES
     ----------

8.1  Each party warrants to the other parties as follows:

     (a)  it has the legal right and power to enter into this Agreement and to
          consummate the transactions contemplated hereby on and subject to the
          terms and conditions of this Agreement, and the execution, delivery
          and performance of this Agreement by it has been duly and validly
          authorised and this Agreement is a valid and binding agreement
          enforceable in accordance with its terms; and

     (b)  no further authorisation, consent or approval of any person is
          required by or in relation to it as a condition to the validity of
          this Agreement or to give effect to the transactions contemplated
          hereby.

9    CONFIDENTIALITY
     ---------------

9.1  All matters relating to this Agreement and the negotiations relating to
     this Agreement and all information acquired or received by any party under
     or in connection with this Agreement shall be held confidential, and each
     party agrees that it shall not divulge any such confidential information to
     any third party, without the prior written approval of all other parties
     provided that any party may, without such approval, disclose such matters
     or information:

     (a)  to any outside professional consultants upon obtaining a similar
          undertaking of confidentiality from such consultants;

     (b)  to any bank or financial institution from whom such party is seeking
          to obtain finance, upon obtaining a similar undertaking of
          confidentiality from such bank or institution;

     (c)  to the extent that the same has become generally available to the
          public other than as a result of unauthorised disclosure by a party;

                                       8

 
     (d)  in the case of a party which is a partnership or a general partner in
          a partnership, to the party's constituent partners; and

     (e)  to persons or the general public if disclosure to such persons or the
          general public is required to comply with any applicable law or
          regulation of any country or the rules or regulations of the London
          Stock Exchange or any other exchange or market on which securities of
          a party or the parent company of a party are quoted, provided that any
          such information disclosed pursuant to this sub-clause (e) shall be
          disclosed only after consultation with the other parties unless such
          consultation is prohibited or the time limits within which such
          disclosure must be made are such that consultation is impracticable.

10   PUBLIC ANNOUNCEMENTS
     --------------------

10.1 No party shall issue or make any public announcements or statements
     regarding this Agreement unless prior thereto such party furnishes all
     parties with a copy of such announcement or statement and obtains the
     approval of the other parties which approval shall not be unreasonably
     withheld provided that, notwithstanding any failure to obtain approval, no
     party shall be prohibited from issuing or making any such public
     announcement or statement if it is necessary to do so in order to comply
     with any applicable law or regulation of any country or the rules or
     regulations of the London Stock Exchange or any other exchange or market on
     which securities of a party or the parent company of a party are quoted, it
     being recognised, however that the parties will endeavour to ensure that
     any such public announcements or statements are made contemporaneously.

11   FURTHER ASSURANCES AND COSTS
     ----------------------------

11.1 The parties shall each execute and deliver such further and other documents
     and instruments and do such further and other things as may be necessary to
     implement and carry out the intent of this Agreement.

11.2 Each party shall bear its own legal and other costs incurred in respect of
     the preparation and negotiation of this Agreement and stamp duty in respect
     of the transfer of the Sale Shares shall be borne by the Purchaser.

12   MODIFICATION AND ASSIGNMENT
     ---------------------------

12.1 No purported variation of this Agreement shall be effective unless made in
     writing and agreed by all the Shareholders.

12.2 No party shall be entitled to assign its rights under this Agreement.

13   EFFECT OF WAIVER
     ----------------

13.1 No waiver by any party of any default in the strict and literal performance
     or compliance with any provision, condition or requirement hereof shall be
     deemed to be a waiver of strict and literal performance of and compliance
     with any other provision, condition or requirement herein nor to be a
     waiver of or in any manner release any other party from strict compliance
     with any provision, condition or requirement in the future.  Nor shall 

                                       9

 
     any delay or omission by any party to exercise any right hereunder in any
     manner impair the exercise of any such right accruing to such party
     thereafter. Except when otherwise expressly stated therein, no remedy
     expressly granted herein to any party shall exclude or be deemed to exclude
     any other remedy which would otherwise be available.

14   ENTIRE AGREEMENT
     ----------------

14.1 This Agreement sets out the entire agreement and understanding between the
     parties with respect to the subject matter hereof and supersedes any prior
     communications or correspondence with respect to the subject matter hereof.
     It is agreed that:

     (i)    no party has entered into this Agreement in reliance upon any
            representation, warranty or undertaking of any other party which is
            not expressly set out or referred to in this Agreement and no party
            shall have any liability for any representation, warranty or
            undertaking of any other party except to the extent (if any)
            expressly set out in this Agreement;

     (ii)   no party shall have any remedy in respect of misrepresentation or
            untrue statement made by any other party unless and to the extent
            that a claim lies for breach of warranty under this Agreement;

     (iii)  this clause shall not exclude any liability for fraudulent
            misrepresentation.

15   GOVERNING LAW AND JURISDICTION
     ------------------------------

15.1 This Agreement shall be governed by and construed and interpreted in
     accordance with the laws of England.

15.2 Each of the parties (for itself and on behalf of its respective holding and
     subsidiary companies and the directors, employees and agents of each of
     them) agrees that the English Courts shall have exclusive jurisdiction to
     hear and decide any and all claims, disputes, complaints, actions or
     proceedings ("Claims or Proceedings"), whether in contract or tort, which
     may arise at any time out of or in connection with any of the matters
     referred to in this Agreement, including, but not limited to, any Claim or
     Proceeding asserting dishonesty, improper or illegal conduct or breach of
     trust or duty or based on the effects of any of those matters in any
     jurisdiction and any Claim or Proceedings which may be material to any of
     the parties but of which any of the parties is unaware or does not suspect
     exists and for this purpose each of the parties irrevocably submits to the
     exclusive jurisdiction of the English Courts.

15.3 CCIC and Berkshire each hereby irrevocably authorise and appoint Norose
     Notices Limited (AMC/99/Z865000) (for the attention of the Director of
     Administration) at the address of its registered office for the time being
     or such other person resident in England as it may by notice to all other
     parties substitute) to accept service of all legal process arising out of
     or connected with this Agreement and service on Norose Notices Limited (or
     such substitute) shall be deemed to be service on the party concerned.

15.4 Each of TdFI and the Purchaser hereby irrevocably authorises and appoints
     Fleetside Legal Representative Services Limited (for the attention of Denis
     Stewart) at the address 

                                      10

 
     of its registered office for the time being (or such other person resident
     in England as it may by notice to all other parties substitute) to accept
     service of all legal process arising out of or connected with this
     Agreement and service on Fleetside Legal Representative Services Limited
     (or such substitute) shall be deemed to be service on the party concerned.

16   NOTICES
     -------

16.1 All notices and other communications required or permitted under this
     Agreement shall be in writing and shall be delivered personally, sent by
     air courier (in the case of notices given by a party in one jurisdiction to
     a party in another), first class pre-paid post (in the case of a notice
     given by a party in one jurisdiction to a party in the same jurisdiction),
     telexed or sent by facsimile transmission (and promptly confirmed by air
     courier service in the case of notices sent from one jurisdiction to
     another and by first class pre-paid post in the case of notices sent by a
     party in one jurisdiction to another party in the same jurisdiction).  Any
     such notice shall be deemed given when so delivered personally, telexed or
     sent by facsimile transmission or air courier or first class pre-paid post
     to the parties at the following addresses (or at such other address for a
     party as shall be specified by like notice):

     Berkshire: if to Berkshire, to:
     ---------                      
     Berkshire Partners
     One Boston Place
     Boston, Massachusetts 02108
     USA
     Attention:    Carl Ferenbach
     Fax:          617-227-6105

     Candover: if to Candover, to:
     --------                     
     Candover Investments PLC
     20 Old Bailey
     London  EC4M 7LN
     Attention:    Douglas Fairservice
     Fax:          0171 248 5483

     CCIC: if to CCIC, to:
     ----                 
     Crown Castle International Corp.
     510 Bering Drive
     Suite 500
     Houston
     Texas TX 77057
     Attention:    Ted B. Miller Jr.
     Fax:          713 570 3150
 
     TdFI or the Purchaser: if to TdFI or the Purchaser to:
     ---------------------
     TeleDiffusion de France International S.A.

                                      11

 
     10 Rue d'Oradour-sur-Glane
     75732 Paris
     Cedex
     France
     Attention:    Michel Azibert
     Fax:          55 95 20 66

     the Company: if to the Company, to:
     -----------                        
     the Company at its registered office
     Attention:    Managing Director
     Fax:          01926 416441

17   COUNTERPARTS
     ------------

     This Agreement may be executed in any number of counterparts with the same
     effect as if the signatures to each such counterparty were upon the same
     instrument.

IN WITNESS of which this agreement has been executed.

                                      12

 
                                  SCHEDULE 1
                                  ----------
                                THE SALE SHARES
                                ---------------


 
        (1)                        (2)             (3)            (4)             (5)
                                                  NO. OF                     CONSIDERATION 
                                                REDEEMABLE   CONSIDERATION    FOR SALE OF
                                                PREFERENCE    FOR SALE OF      REDEEMABLE
                            NO. OF ORDINARY    SHARES TO BE     ORDINARY       PREFERENCE
   NAME OF SELLER          SHARES TO BE SOLD       SOLD          SHARES          SHARES
                                                                (POUNDS)        (POUNDS)
                                                                   
Berkshire Fund IV,               490,909       490,418,231    7,368,544.09    4,904,182.31
LP                                        
Berkshire Investors               49,091        49,041,769      736,855.91      490,417.69
LLC                                       
Candover Investments             217,699       241,646,116    3,267,661.99    2,416,461.16
plc                                       
Candover (Trustees)               24,189                 0      363,076.89               0
Limited                                   
Candover Partners                438,550       438,111,904    6,582,635.50    4,381,119.04
Limited (as general                       
partner of the                            
Candover 1994 UK                          
Limited Partnership)                      
Candover Partners                118,847       118,728,160    1,783,893.47    1,187,281.60
Limited (as general                       
partner of the                            
Candover 1994 UK                          
No. 2 Limited                             
Partnership)                              
Candover Partners                 21,286        21,264,745      319,502.86      212,647.45
Limited (as general                       
partner of the                            
Candover 1994 US                          
No. 1 Limited                             
Partnership)                              
Candover Partners                259,429       259,169,075    3,894,029.29    2,591,690.75
Limited (as general
partner of the
Candover 1994 US
No. 2 Limited
Partnership)


                                      13

 
                                  SCHEDULE 2
                                  ----------
                               DEED OF ADHERENCE
                               -----------------

THIS DEED OF ADHERENCE is made on                                      1998 
BETWEEN:

[INSERT NAME OF THE PURCHASER] of [ADDRESS] (the "NEW SHAREHOLDER") in favour of
the persons whose names are set out in the schedule to this deed and is
supplemental to the Shareholders' Agreement dated 23 January 1997 between
Berkshire Fund IV Investment Corp and others (as amended by a Deed of Adherence
dated 2 May 1997 and an agreement dated . 1998) (together the "AGREEMENT").

THE PARTIES AGREE AS FOLLOWS:

1    The New Shareholder confirms that it has read a copy of the Agreement and
     covenants with each person named in the schedule to this deed to perform
     and be bound by all the terms of the agreement as if the New Shareholder
     were a party thereto and fell within the scope of the definition "TDFI"
     together with TeleDiffusion de France International S.A.

2    This deed is governed by English law.

IN WITNESS whereof this deed has been executed by the New Shareholder and is
intended to be and is hereby delivered on the date first above written.

                                      14

 
                                   SCHEDULE
                                   --------

(1)  BERKSHIRE FUND IV, LP, a Massachusetts limited partnership;

(2)  BERKSHIRE PARTNERS LLC, a Massachusetts limited liability corporation;

(3)  CANDOVER INVESTMENTS PLC, a company incorporated in England and Wales;

(4)  CANDOVER (TRUSTEES) LIMITED, a company incorporated in England and Wales;

(5)  CANDOVER PARTNERS LIMITED, a company incorporated in England and Wales (as
     general partner of the Candover 1994 UK Limited Partnership);

(6)  CANDOVER PARTNERS LIMITED, a company incorporated in England and Wales (as
     general partner of the Candover 1994 UK No. 2 Limited Partnership);

(7)  CANDOVER PARTNERS LIMITED, a company incorporated in England and Wales (as
     general partner of the Candover 1994 US No. 1 Limited Partnership);

(8)  CANDOVER PARTNERS LIMITED, a company incorporated in England and Wales (as
     general partner of the Candover 1994 US No. 2 Limited Partnership);

(9)  CROWN CASTLE INTERNATIONAL CORP., a Delaware corporation;

(10) TELEDIFFUSION DE FRANCE INTERNATIONAL S.A., a company incorporated in
     France;

(11) CASTLE TRANSMISSION SERVICES (HOLDINGS) LTD (formerly known as Diohold
     Limited), a company incorporated in England and Wales.

                                      15

 
                                  SCHEDULE 3
                                  ----------
                                    OPTIONS
                                    -------


1    INTERPRETATION
     --------------

1.1  In this schedule unless the context otherwise requires:

     "BERKSHIRE OPTION SECURITIES" means all Ordinary Shares, Redeemable
     Preference Shares and other securities of the Company held by Berkshire on
     the date on which either of the Options is exercised;

     "CANDOVER OPTION SECURITIES" means all Ordinary Shares, Redeemable
     Preference Shares and other securities of the Company held by Candover on
     the date on which either of the Options is exercised;

     "CCIC RELEVANT PROPORTION" means (a) if both the Options shall be
     exercised, such fraction as will, following completion of the exercise of
     the Options, result in CCIC and TdFI holding the same number of Ordinary
     Shares and Redeemable Preference Shares of the Company and (b) if one only
     of the Options shall be exercised, a fraction calculated by reference to
     the following formula:
           
          B
     A =  -
          C 
 
     Where:

     "A" is the relevant fraction;

     "B" is equal to the number of Ordinary Shares of the Company held by CCIC
     at the date of the exercise of such Option; and

     "C" is equal to the aggregate of the number of Ordinary Shares of the
     Company held by CCIC and TdFI at the date of the exercise of such Option;

     Provided that, if the fraction calculated by the above formula would result
     in CCIC in any circumstances coming to hold more than 50 per cent. of the
     issued share capital of the Company, the CCIC Relevant Proportion shall be
     such fraction as will result in CCIC holding no more than 50 per cent. of
     the issued share capital of the Company and TdFI shall be required to
     acquire such number of Ordinary Shares and Redeemable Preference Shares as
     CCIC would otherwise have been required to acquire but for this proviso.

     "OPTION PERIOD" means the period commencing on 1 March 2002 and ending on
     14 April 2002;

     "OPTIONS" means the options granted by paragraph 2.1;

                                      16

 
     "PURCHASE CONSIDERATION" means an amount determined in accordance with
     paragraph 3;

     "TDFI RELEVANT PROPORTIONS" means (a) if both the Options shall be
     exercised, such fraction as will, following completion of the exercise of
     the Options, result in CCIC and TdFI holding the same number of Ordinary
     Shares and Redeemable Preference Shares of the Company and (b) if one only
     of the Options shall be exercised, a fraction calculated by reference to
     the following formula:

          B 
     A =  - 
          C  

     Where:

     "A" is the relevant fraction;

     "B" is equal to the number of Ordinary Shares of the Company held by TdFI
     at the date of the exercise of such Option; and

     "C" is equal to the aggregate of the number of Ordinary Shares of the
     Company held by CCIC and TdFI at the date of the exercise of such Option;

     Provided that, if the fraction calculated by the above formula would result
     in TdFI in any circumstances coming to hold more than 50 per cent. of the
     issued share capital of the Company, the TdFI Relevant Proportion shall be
     such fraction as will result in TdFI holding no more than 50 per cent. of
     the issued share capital of the Company and CCIC shall be required to
     acquire such number of Ordinary Shares and Redeemable Preference Shares as
     TdFI would otherwise have been required to acquire but for this proviso.

2    GRANT OF OPTIONS
     ----------------

2.1  Subject to paragraph 2.2 and in consideration of (Pounds)1 now paid by each
     of Candover and Berkshire (receipt of which is hereby acknowledged):

     (a)  CCIC hereby grants to Berkshire an option to require CCIC to purchase
          the CCIC Relevant Proportion of each class of the Berkshire Option
          Securities;

     (b)  CCIC hereby grants to Candover an option to require CCIC to purchase
          the CCIC Relevant Proportion of each class of the Candover Option
          Securities;

     (c)  TdFI hereby grants to Berkshire an option to require TdFI to purchase
          the TdFI Relevant Proportion of each of class of the Berkshire Option
          Securities; and

     (d)  TdFI hereby grants to Candover an option to require TdFI to purchase
          the TdFI Relevant Proportion of each class of the Candover Option
          Securities,

     in each case on the terms of this Agreement.  The Options contained in
     paragraphs 2.1(a) and (c) may not be exercised independently of one another
     and the Options 

                                      17

 
     contained in paragraphs 2.1(b) and (d) may not be exercised independently
     of one another.

2.2  The Options shall lapse if prior to the commencement of the Option Period
     any Ordinary Shares of the Company shall be listed on the official list of
     London Stock Exchange Limited or any other stock exchange or if permission
     is granted for any Ordinary Shares of the Company to be dealt in on any
     other stock exchange.

2.3  Berkshire hereby undertakes that the Berkshire Option Securities which are
     the subject of an exercise of either of the Options shall be sold by it
     free from all liens, charges, encumbrances and adverse interests or claims
     of any person and, subject thereto, with full title guarantee, but no other
     warranties, representations or assurances shall be given in respect of the
     Berkshire Option Securities.

2.4  Candover hereby undertakes that the Candover Option Securities which are
     the subject of an exercise of either of the Options shall be sold by it
     free from all liens, charges, encumbrances and adverse interests or claims
     of any person and, subject thereto, with full title guarantee, but no other
     warranties, representations or assurances shall be given in respect of the
     Candover Option Securities.

2.5  Each of the parties hereby waives any rights of pre-emption conferred on it
     by the Articles of Association of the Company or by the Shareholders'
     Agreement or otherwise over any Berkshire Option Securities and/or Candover
     Option Securities which may become subject to the exercise of either of the
     Options or of any options granted under the terms of the agreement set out
     in schedule 5 hereto.

3    EXERCISE OF THE OPTIONS
     -----------------------

3.1  Each of Berkshire and Candover may give notice to exercise the Options
     during the Option Period.  Such notice shall specify a date (being a week
     day) on which (subject to determination of the Purchase Consideration) the
     exercise of the relative option shall be completed, which date shall be not
     more than three months nor less than two months after the date of the
     notice.

3.2  The parties agree that the Purchaser first named above (the "ORIGINAL
     PURCHASER") shall have the right to nominate in its place as Purchaser
     hereunder any other undertaking which, for the purposes of the Companies
     Act 1985 (as amended), is a subsidiary of the Original Purchaser, a holding
     company of the Original Purchaser or a subsidiary of any such holding
     company.  If written notice of such nomination is given by the Original
     Purchaser (prior to the exercise of any Options) to the other parties
     hereto, such replacement Purchaser (the "REPLACEMENT PURCHASER") shall
     forthwith agree to become a party to this Agreement by executing such deed
     of accession as the other parties may reasonably require and, upon
     execution of such deed, shall be entitled to all of the rights, and be
     bound by all of the obligations, of the Original Party hereunder, provided
     however that Original Purchaser shall continue to be liable to discharge
     such obligations to the extent that such Replacement Purchaser shall fail
     so to do.

                                      18

 
4    THE PURCHASE CONSIDERATION
     --------------------------

4.1  The Purchase Consideration for the Berkshire Option Securities or, as the
     case may be, the Candover Option Securities shall be the fair market value
     thereof (valued as a proportion of the value of the Company as a whole
     which is equal to the proportion of the entire issued share capital of the
     Company represented by the Berkshire Option Securities or, as the case may
     be, the Candover Option Securities) as determined by an appraisal of the
     value of the shares in question performed by a mutually acceptable
     investment banking firm that has not been engaged on a regular basis by the
     Company, CCIC, TdFI, Berkshire or Candover within the two years preceding
     the exercise of the relative option, with experience of giving appraisals
     in the financial analysis of businesses similar to the Company's business.
     If the prospective seller and the prospective purchaser are unable to agree
     on a mutually acceptable investment banking or other firm within 7 days
     following the exercise of the relative option, the prospective seller and
     the prospective purchaser shall select its own investment banking or other
     firm and the two selected firms shall select a mutually acceptable
     investment banking or other firm (meeting the criteria set forth above)
     and, in default of agreement, such investment banking or other firm shall
     be nominated on the application of either the prospective seller or the
     prospective seller by the Chairman of the New York Stock Exchange, Inc.  If
     either party fails to select its own investment banking or other firm to
     select the determining firm within five days following the expiration of
     such seven day period, the other party's selected firm shall act as the
     determining firm.  Upon selection of a firm pursuant to the foregoing
     procedures, such firm shall be provided with such financial information
     concerning, and access to, the Company or such parties as shall be
     reasonably requested by such firm in connection with its determination,
     including attending meetings with, or making presentations to, such firm.
     The selected firm shall deliver to the applicable parties a final
     determination of the Purchase Consideration within 30 days of its
     engagement, and the parties hereby agree to be bound by such decision.  The
     fees and expenses of the determining firm shall be borne by the prospective
     seller and the prospective purchaser in equal shares.  The applicable
     parties shall be responsible for their own fees and expenses, including the
     fees and expenses of their respective lawyers, and, if applicable, their
     own investment banking or other firm.

5    COMPLETION
     ----------

5.1  Completion of the exercise of the Options shall take place at the
     registered office of the Company on the date specified in the notice
     mentioned in clause 3 (or, if later, 7 days after the Purchase
     Consideration shall have been finally determined) when all (but not part
     only) of the following business shall be transacted:

     (a)  CCIC shall pay to Berkshire or, as the case may be, Candover the CCIC
          Relevant Proportion of the Purchase Consideration and TdFI shall pay
          to Berkshire or, as the case may be, Candover the TdFI Relevant
          Proportion of the Purchase Consideration;

                                      19

 
     (b)  Berkshire or, as the case may be, Candover shall deliver to CCIC
          transfers in respect of the CCIC Relevant Proportion of each class of
          the Berkshire Option Securities or, as the case may be, the Candover
          Option Securities duly completed in favour of CCIC together with the
          certificate therefor (or, failing such certificate, an indemnity in
          terms which would be satisfactory to a reasonable purchaser);

     (c)  Berkshire or, as the case may be, Candover shall deliver to TdFI
          transfers in respect of the TdFI Relevant Proportion of each class of
          the Berkshire Option Securities or, as the case may be, the Candover
          Option Securities duly completed in favour of TdFI together with the
          certificates therefor (or, failing such certificates, an indemnity in
          terms which would be satisfactory to a reasonable purchaser).

5.2  If either CCIC or TdFI shall default in its obligation to make payment in
     accordance with paragraph 5.1(a) and shall fail to remedy such default
     within 7 days, then such party shall lose its rights under this schedule 3
     and whichever of CCIC or TdFI which has not so defaulted shall be entitled
     (by notice in writing to Berkshire or, as the case may be, Candover given
     within 5 days after the expiry of such 7 day period) elect to complete the
     exercise of the relevant Option in place of the party who so defaulted and
     the provisions of paragraph 5.1 shall apply mutatis mutandis.

6    STANDBY OPTION
     --------------

6.1  The parties shall join in procuring that, as soon as practicable after the
     date hereof, an Extraordinary General Meeting of the Company shall be
     convened for the purpose of considering the resolution set out in schedule
     4 as a special resolution, and each of the parties hereto (other than the
     Company) undertakes to vote in favour of such resolution.

6.2  Subject to the resolution referred to in paragraph 6.1 being duly passed,
     the Company, Candover and Berkshire each undertake to enter into the
     agreement set out in schedule 5 not later than 5 days after the date on
     which such resolution is passed.

7    ASSIGNMENT
     ----------

     None of the parties shall be entitled to assign or transfer its rights
     under this schedule.

                                      20

 
                                  SCHEDULE 4
                                  ----------
                                THE RESOLUTION
                                --------------

THAT the Company be and it is hereby authorised to make an off-market purchase
of its own shares in accordance with section 165 Companies Act 1985 on the terms
set out in the draft Standby Option Agreement between Berkshire Fund IV, LP (1),
Berkshire Investors LLC (2), Berkshire Partners LLC (3), Candover Investments
PLC (4), Candover (Trustees) Limited (5), Candover Partners Limited (6),
Candover Partners Limited (7), Candover Partners Limited (8), Candover Partners
Limited (9) and Castle Transmission Services (Holdings) Limited (10) in the form
produced to the meeting.

                                      21

 
                                  SCHEDULE 5
                                  ----------
                            STANDBY OPTION AGREEMENT
                            ------------------------



                              BERKSHIRE FUND IV, LP             (1)
                             BERKSHIRE INVESTORS LLC            (2)
                              BERKSHIRE PARTNERS LLC            (3)
                             CANDOVER INVESTMENTS PLC           (4)
                           CANDOVER (TRUSTEES) LIMITED          (5)
                            CANDOVER PARTNERS LIMITED           (6)
                            CANDOVER PARTNERS LIMITED           (7)
                            CANDOVER PARTNERS LIMITED           (8)
                            CANDOVER PARTNERS LIMITED           (9)
             CASTLE TRANSMISSION SERVICES (HOLDINGS) LIMITED   (10)


                      _________________________________ 

                            STANDBY OPTION AGREEMENT

                      _________________________________ 



                                  NORTON ROSE
                                    London

                                      22

 
THIS AGREEMENT is dated
 .............................................................. 1998 and is made
AMONG:

(1)  BERKSHIRE FUND IV, LP, a Massachusetts limited partnership;

(2)  BERKSHIRE INVESTORS LLC, a Massachusetts limited liability corporation;

(3)  BERKSHIRE PARTNERS LLC, a Massachusetts limited liability corporation;

(4)  CANDOVER INVESTMENTS PLC, a company incorporated in England and Wales;

(5)  CANDOVER (TRUSTEES) LIMITED, a company incorporated in England and Wales;

(6)  CANDOVER PARTNERS LIMITED, a company incorporated in England and Wales (as
     general partner of the Candover 1994 UK Limited Partnership);

(7)  CANDOVER PARTNERS LIMITED, a company incorporated in England and Wales (as
     general partner of the Candover 1994 UK No. 2 Limited Partnership);

(8)  CANDOVER PARTNERS LIMITED, a company incorporated in England and Wales (as
     general partner of the Candover 1994 US No. 1 Limited Partnership);

(9)  CANDOVER PARTNERS LIMITED, a company incorporated in England and Wales (as
     general partner of the Candover 1994 US No. 2 Limited Partnership);

(10) CASTLE TRANSMISSION SERVICES (HOLDINGS) LIMITED (the "COMPANY"), a company
     incorporated in England and Wales.

1    DEFINITIONS AND INTERPRETATION
     ------------------------------

1.1  In this Agreement unless the context otherwise requires:

     "BERKSHIRE" has the meaning ascribed to such expression in the
     Shareholders' Agreement;

     "CANDOVER" has the meaning ascribed to such expression in the Shareholders'
     Agreement;

     "SHAREHOLDERS' AGREEMENT" means the shareholders' agreement in respect of
     the Company dated 23 January 1997 (as amended by a Deed of Adherence dated
     2 May 1997);

     "BERKSHIRE OPTION SECURITIES" means all Ordinary Shares, Redeemable
     Preference Shares and other securities of the Company held by Berkshire on
     the date on which either of the Standby Options is exercised;

     "CANDOVER OPTION SECURITIES" means all Ordinary Shares, Redeemable
     Preference Shares and other securities of the Company held by Candover on
     the date on which either of the Standby Options is exercised;

     "STANDBY OPTIONS" means the options granted by clause 2.1;

                                      23

 
     "PURCHASE CONSIDERATION" means an amount determined in accordance with
     clause 3;

     "STANDBY OPTION PERIOD" means the period commencing on the 13/th/ day
     following the date (if any) on which Crown Castle International Corp. or,
     as the case may be, TeleDiffusion de France International S.A. fails (in
     breach of its contractual obligations) to complete the purchase of the
     Berkshire Option Securities or, as the case may be, the Candover Option
     Securities under the terms of schedule 3 to the Share Sale Agreement
     pursuant to an exercise of either of the Options (as defined in the Share
     Sale Agreement) in circumstances in which Crown Castle International Corp.
     (if it is not the party which so breached its contractual obligations) or
     TeleDiffusion de France International S.A. (if it is not the party which so
     breached its contractual obligations) has not elected to exercise its right
     to purchase the Berkshire Option Securities or, as the case may be, the
     Candover Option Securities pursuant to paragraph 5.2 of schedule 3 to the
     Share Sale Agreement (or, having exercised such right, fails to comply with
     its obligations under paragraph 5.2 of schedule 3 to the Share Sale
     Agreement) and ending 45 days after the commencement of such period;

     "SHARE SALE AGREEMENT" means an agreement dated 24 April 1998 between
     Berkshire Fund IV, LP and others for the sale and purchase of certain
     shares of the Company, for the amendment of the Shareholders' Agreement and
     for the granting of certain options.

1.2  In this Agreement unless the context otherwise requires:

     (a)  a document expressed to be "IN THE AGREED FORM" means a document in a
          form which has been agreed by the parties contemporaneously with or
          before the execution of this Agreement and which has, for the purposes
          of identification, been signed or initialled by them or on their
          behalf;

     (b)  references to a clause or schedule are to a clause of, or a schedule
          to, this Agreement, references to this Agreement include its schedules
          and references in a schedule or part of a schedule to a paragraph are
          to a paragraph of that schedule or that part of that schedule;

     (c)  references to this Agreement or any other document or to any specified
          provision of this Agreement or any other document are to this
          Agreement, that document or that provision as in force for the time
          being and as amended from time to time in accordance with the terms of
          this Agreement or that document or, as the case may be, with the
          agreement of the relevant parties;

     (d)  words importing the singular include the plural and vice versa, words
          importing a gender include every gender and references to persons
          include corporations, partnerships and other unincorporated
          associations or bodies of persons;

     (e)  the contents table and the descriptive headings to clauses, schedules
          and paragraphs are inserted for convenience only, have no legal effect
          and shall be ignored in the interpretation of this Agreement.

                                      24

 
2    GRANT OF OPTIONS
     ----------------

2.1  Subject to paragraph 2.2, the Company (in consideration of (Pounds)1 now
     paid by each of Candover and Berkshire, of which receipt is hereby
     acknowledged) hereby grants:

     (a)  to Berkshire an option to require the Company to purchase the
          Berkshire Option Securities; and

     (b)  to Candover an option to require the Company to purchase the Candover
          Option Securities,

     in each case on the terms of this Agreement.

2.2  The Standby Options shall lapse if:

     (a)  prior to the commencement of the Option Period (as defined in the
          Share Sale Agreement) any Ordinary Shares of the Company shall be
          listed on the official list of London Stock Exchange Limited or any
          other stock exchange or if permission is granted for any Ordinary
          Shares of the Company to be dealt in on any other stock exchange; or

     (b)  the Company shall notify the party exercising either of the Standby
          Options within 35 days of the exercise thereof that it is unable to
          complete the purchase of the Berkshire Option Securities or, as the
          case may be, the Candover Option Securities:

          (i)  by reason of its inability to comply with the relevant provisions
               of Companies Act 1985 (or any other legislation then in force) in
               respect of such purchase; or

          (ii) without being in breach of the terms of the (Pounds)125,000,000 9
               per cent. Guaranteed Bonds due 2007 issued by Castle Transmission
               (Finance) PLC or the terms of any of its other financing
               facilities.

2.3  Berkshire hereby undertakes that the Berkshire Option Securities which are
     the subject of an exercise of the Standby Options shall be sold by it free
     from all liens, charges, encumbrances and adverse interests or claims of
     any person and, subject thereto, with full title guarantee, but no other
     warranties, representations or assurances shall be given in respect of the
     Berkshire Option Securities.

2.4  Candover hereby undertakes that the Candover Option Securities which are
     the subject of an exercise of the Standby Options shall be sold by it free
     from all liens, charges, encumbrances and adverse interests or claims of
     any person and, subject thereto, with full title guarantee, but no other
     warranties, representations or assurances shall be given in respect of the
     Candover Option Securities.

3    EXERCISE OF THE STANDBY OPTIONS
     -------------------------------

3.1  Each of Berkshire and Candover may give notice to exercise the Standby
     Options during the Standby Option Period.  Such notice shall specify a date
     (being a week day) on which (subject to determination of the Purchase
     Consideration) the exercise of the 

                                      25

 
     relative option shall be completed, which date shall be not more than three
     months nor less than two months after the date of the notice.

4    THE PURCHASE CONSIDERATION
     --------------------------

4.1  The Purchase Consideration for the Berkshire Option Securities or, as the
     case may be, the Candover Option Securities shall be the fair market value
     thereof as determined pursuant to paragraph 4.1 of schedule 3 to the Share
     Sale Agreement.

5    COMPLETION
     ----------

5.1  Completion of the exercise of the Standby Options shall take place at the
     registered office of the Company on the date specified in the notice
     mentioned in clause 3 when all (but not part only) of the following
     business shall be transacted:

     (a)  the Company shall pay to Berkshire or, as the case may be, Candover
          the Purchase Consideration;

     (b)  the Company shall deliver to Berkshire or, as the case may be,
          Candover evidence that it has duly complied with all the requirements
          of the Companies Act 1985 and of all other requirements of legislation
          then in force in respect of the purchase;

     (c)  Berkshire or, as the case may be, Candover shall deliver to the
          Company transfers in respect of the Berkshire Option Securities or, as
          the case may be, the Candover Option Securities duly completed in
          favour of the Company together with the certificates therefor (or,
          failing such certificates, an indemnity in terms which would be
          satisfactory to a reasonable purchaser).

6    WARRANTIES
     ----------

6.1  Each party warrants to the other parties as follows:

     (a)  it has the legal right and power to enter into this Agreement and to
          consummate the transactions contemplated hereby on and subject to the
          terms and conditions of this Agreement, and the execution, delivery
          and performance of this Agreement by it has been duly and validly
          authorised and this Agreement is a valid and binding agreement
          enforceable in accordance with its terms; and

     (b)  no further authorisation, consent or approval of any person is
          required by or in relation to it as a condition to the validity of
          this Agreement or to give effect to the transactions contemplated
          hereby.

7    CONFIDENTIALITY
     ---------------

7.1  All matters relating to this Agreement and the negotiations relating to
     this Agreement and all information acquired or received by any party under
     or in connection with this Agreement shall be held confidential, and each
     party agrees that it shall not divulge any such confidential information to
     any third party, without the prior written 

                                      26

 
     approval of all other parties provided that any party may, without such
     approval, disclose such matters or information:

     (a)  to any outside professional consultants upon obtaining a similar
          undertaking of confidentiality from such consultants;

     (b)  to any bank or financial institution from whom such party is seeking
          to obtain finance, upon obtaining a similar undertaking of
          confidentiality from such bank or institution;

     (c)  to the extent that the same has become generally available to the
          public other than as a result of unauthorised disclosure by a party;

     (d)  in the case of a party which is a partnership or a general partner in
          a partnership, to the party's constituent partners; and

     (e)  to persons or the general public if disclosure to such persons or the
          general public is required to comply with any applicable law or
          regulation of any country or the rules or regulations of the London
          Stock Exchange or any other exchange or market on which securities of
          a party or the parent company of a party are quoted, provided that any
          such information disclosed pursuant to this sub-clause (e) shall be
          disclosed only after consultation with the other parties unless such
          consultation is prohibited or the time limits within which such
          disclosure must be made are such that consultation is impracticable.

8    PUBLIC ANNOUNCEMENTS
     --------------------

8.1  No party shall issue or make any public announcements or statements
     regarding this Agreement unless prior thereto such party furnishes all
     parties with a copy of such announcement or statement and obtains the
     approval of the other parties which approval shall not be unreasonably
     withheld provided that, notwithstanding any failure to obtain approval, no
     party shall be prohibited from issuing or making any such public
     announcement or statement if it is necessary to do so in order to comply
     with any applicable law or regulation of any country or the rules or
     regulations of the London Stock Exchange or any other exchange or market on
     which securities of a party or the parent company of a party are quoted, it
     being recognised, however that the parties will endeavour to ensure that
     any such public announcements or statements are made contemporaneously.

9    FURTHER ASSURANCES AND COSTS
     ----------------------------

9.1  The parties shall each execute and deliver such further and other documents
     and instruments and do such further and other things as may be necessary to
     implement and carry out the intent of this Agreement.

9.2  Each party shall bear its own legal and other costs incurred in respect of
     the preparation and negotiation of this Agreement and stamp duty in respect
     of the transfer of the Berkshire Option Securities and the Candover Option
     Securities shall be borne by the Company.

                                      27

 
10   MODIFICATION AND ASSIGNMENT
     ---------------------------

10.1 No purported variation of this Agreement shall be effective unless made in
     writing and agreed by all the parties hereto.

10.2 No party shall be entitled to assign its rights under this Agreement.

11   EFFECT OF WAIVER
     ----------------

11.1 No waiver by any party of any default in the strict and literal performance
     or compliance with any provision, condition or requirement hereof shall be
     deemed to be a waiver of strict and literal performance of and compliance
     with any other provision, condition or requirement herein nor to be a
     waiver of or in any manner release any other party from strict compliance
     with any provision, condition or requirement in the future.  Nor shall any
     delay or omission by any party to exercise any right hereunder in any
     manner impair the exercise of any such right accruing to such party
     thereafter. Except when otherwise expressly stated therein, no remedy
     expressly granted herein to any party shall exclude or be deemed to exclude
     any other remedy which would otherwise be available.

12   ENTIRE AGREEMENT
     ----------------

12.1 This Agreement sets out the entire agreement and understanding between the
     parties with respect to the subject matter hereof and supersedes any prior
     communications or correspondence with respect to the subject matter hereof.
     It is agreed that:

     (i)    no party has entered into this Agreement in reliance upon any
            representation, warranty or undertaking of any other party which is
            not expressly set out or referred to in this Agreement and no party
            shall have any liability for any representation, warranty or
            undertaking of any other party except to the extent (if any)
            expressly set out in this Agreement;

     (ii)   no party shall have any remedy in respect of misrepresentation or
            untrue statement made by any other party unless and to the extent
            that a claim lies for breach of warranty under this Agreement;

     (iii)  this clause shall not exclude any liability for fraudulent
            misrepresentation.

13   GOVERNING LAW AND JURISDICTION
     ------------------------------

13.1 This Agreement shall be governed by and construed and interpreted in
     accordance with the laws of England.

13.2 Each of the parties (for itself and on behalf of its respective holding and
     subsidiary companies and the directors, employees and agents of each of
     them) agrees that the English Courts shall have exclusive jurisdiction to
     hear and decide any and all claims, disputes, complaints, actions or
     proceedings ("Claims or Proceedings"), whether in contract or tort, which
     may arise at any time out of or in connection with any of the matters
     referred to in this Agreement, including, but not limited to, any Claim or
     Proceeding asserting dishonesty, improper or illegal conduct or breach of
     trust or duty 

                                      28

 
     or based on the effects of any of those matters in any jurisdiction and any
     Claim or Proceedings which may be material to any of the parties but of
     which any of the parties is unaware or does not suspect exists and for this
     purpose each of the parties irrevocably submits to the exclusive
     jurisdiction of the English Courts.

13.3 Berkshire hereby irrevocably authorises and appoints Norose Notices Limited
     (AMC/99/Z865000) (for the attention of the Director of Administration) at
     the address of its registered office for the time being or such other
     person resident in England as it may by notice to all other parties
     substitute) to accept service of all legal process arising out of or
     connected with this Agreement and service on Norose Notices Limited (or
     such substitute) shall be deemed to be service on the party concerned.

14   NOTICES
     -------

14.1 All notices and other communications required or permitted under this
     Agreement shall be in writing and shall be delivered personally, sent by
     air courier (in the case of notices given by a party in one jurisdiction to
     a party in another), first class pre-paid post (in the case of a notice
     given by a party in one jurisdiction to a party in the same jurisdiction),
     telexed or sent by facsimile transmission (and promptly confirmed by air
     courier service in the case of notices sent from one jurisdiction to
     another and by first class pre-paid post in the case of notices sent by a
     party in one jurisdiction to another party in the same jurisdiction).  Any
     such notice shall be deemed given when so delivered personally, telexed or
     sent by facsimile transmission or air courier or first class pre-paid post
     to the parties at the following addresses (or at such other address for a
     party as shall be specified by like notice):

     Berkshire: if to Berkshire, to:
     ---------                      
     Berkshire Partners
     One Boston Place
     Boston, Massachusetts 02108
     USA
     Attention:    Carl Ferenbach
     Fax:          617-227-6105
 
     Candover: if to Candover, to:
     --------
     Candover Investments PLC
     20 Old Bailey
     London  EC4M 7LN
     Attention:    Douglas Fairservice
     Fax:          0171 248 5483

     the Company: if to the Company, to:
     -----------                        
     the Company at its registered office
     Attention:    Managing Director
     Fax:          01926 416441

                                      29

 
15   COUNTERPARTS
     ------------

     This Agreement may be executed in any number of counterparts with the same
     effect as if the signatures to each such counterparty were upon the same
     instrument.

IN WITNESS of which this agreement has been executed.

                                      30

 
SIGNED for and on behalf of     )
BERKSHIRE FUND IV, LP           )
By                              )
in the presence of:             )


SIGNED for and on behalf of     )
BERKSHIRE INVESTORS LLC         )
By                              )
in the presence of:             )


SIGNED for and on behalf of     )
BERKSHIRE PARTNERS LLC          )
By                              )
in the presence of:             )


SIGNED for and on behalf of     )
CANDOVER INVESTMENTS PLC        )
By                              )
in the presence of:             )


SIGNED for and on behalf of     )
CANDOVER (TRUSTEES) LIMITED     )
By                              )
in the presence of:             )


SIGNED for and on behalf of     )
CANDOVER PARTNERS LIMITED       )
(as general partner of the      )
Candover 1994 UK Limited        )
Partnership)                    )
By                              )
in the presence of:             )

                                      31

 
SIGNED for and on behalf of          )
CANDOVER PARTNERS LIMITED            )
(as general partner of the           )
Candover 1994 UK No. 2 Limited       )
Partnership)                         )
By                                   )
in the presence of:                  )



SIGNED for and on behalf of          )
CANDOVER PARTNERS LIMITED            )
(as general partner of the           )
Candover 1994 US No. 1 Limited       )
Partnership)                         )
By                                   )
in the presence of:                  )


SIGNED for and on behalf of          )
CANDOVER PARTNERS LIMITED            )
(as general partner of the           )
Candover 1994 US No. 2 Limited       )
Partnership)                         )
By                                   )
in the presence of:                  )


SIGNED for and on behalf of          )
CASTLE TRANSMISSION                  )
SERVICES (HOLDINGS) LIMITED          )
By                                   )
in the presence of:                  )

                                      32

 
SIGNED for and on behalf of     )
BERKSHIRE FUND IV, LP           )
By                              )
in the presence of:             )


SIGNED for and on behalf of     )
BERKSHIRE INVESTORS LLC         )
By                              )
in the presence of:             )


SIGNED for and on behalf of     )
BERKSHIRE PARTNERS LLC          )
By                              )
in the presence of:             )


SIGNED for and on behalf of     )
CANDOVER INVESTMENTS PLC        )
By                              )
in the presence of:             )


SIGNED for and on behalf of     )
CANDOVER (TRUSTEES) LIMITED     )
By                              )
in the presence of:             )


SIGNED for and on behalf of     )
CANDOVER PARTNERS LIMITED       )
(as general partner of the      )
Candover 1994 UK Limited        )
Partnership)                    )
By                              )
in the presence of:             )

                                      33

 
SIGNED for and on behalf of          )
CANDOVER PARTNERS LIMITED            )
(as general partner of the           )
Candover 1994 UK No. 2 Limited       )
Partnership)                         )
By                                   )
in the presence of:                  )



SIGNED for and on behalf of          )
CANDOVER PARTNERS LIMITED            )
(as general partner of the           )
Candover 1994 US No. 1 Limited       )
Partnership)                         )
By                                   )
in the presence of:                  )


SIGNED for and on behalf of          )
CANDOVER PARTNERS LIMITED            )
(as general partner of the           )
Candover 1994 US No. 2 Limited       )
Partnership)                         )
By                                   )
in the presence of:                  )


SIGNED for and on behalf of          )
CROWN CASTLE INTERNATIONAL           )
CORP.                                )
By:                                  )
in the presence of:                  )


SIGNED for and on behalf of          )
TELEDIFFUSION DE FRANCE              )
INTERNATIONAL S.A.                   )
By                                   )
in the presence of:                  )


SIGNED for and on behalf of          )
DIGITAL FUTURE INVESTMENTS B.V.      )

                                      34

 
By                                   )
in the presence of:                  )


SIGNED for and on behalf of          )
CASTLE TRANSMISSION                  )
SERVICES (HOLDINGS) LIMITED          )
By                                   )
in the presence of:                  )

                                      35