FIRST SUPPLEMENTAL INDENTURE (the "Supplemental  Indenture"),  dated as
of  September  27,  2000,  among  Trenwick  Group Inc.,  a Delaware  corporation
("Trenwick"),  Trenwick America Corporation,  a Delaware corporation  ("Trenwick
America") and The Chase Manhattan Bank, as trustee under the Indenture  referred
to below (the "Trustee").
                               W I T N E S S E T H

         WHEREAS,  pursuant  to the  Indenture  (the  "Indenture"),  dated as of
January 31, 1997,  between  Trenwick and the Trustee,  Trenwick issued its 8.82%
Junior   Subordinated   Deferrable  Interest   Debentures   (collectively,   the
"Securities");

         WHEREAS,  Trenwick, LaSalle Re Holdings Limited, LaSalle Re Limited and
Trenwick Group Ltd. (formerly Gowin Holdings International Limited) have entered
into an Amended  and  Restated  Agreement,  Schemes of  Arrangement  and Plan of
Reorganization,  dated as of March 20, 2000, in connection  with which  Trenwick
will transfer its property substantially as an entirety to Chartwell Re Holdings
Corporation, a Delaware corporation ("Chartwell Re") and Chartwell Re will merge
with and intoTrenwick America (the "Asset Transfer");

         WHEREAS,  upon  consummation of the Asset  Transfer,  all of the debts,
liabilities,   duties  and  obligations  of  Trenwick  will  become  the  debts,
liabilities,  duties and  obligations  of Trenwick  America,  including  without
limitation the debts, liabilities,  duties and obligations of Trenwick under the
Indenture;

         WHEREAS, Section 10.01 of the Indenture provides in pertinent part that
Trenwick  may  consummate  the Asset  Transfer and that,  in the event  thereof,
Trenwick America shall expressly assume, by supplemental indenture, executed and
delivered  to  the  Trustee,  all of  the  obligations  of  Trenwick  under  the
Indenture;

         WHEREAS, Section 10.03 of the Indenture provides in pertinent part that
in connection  with the Asset  Transfer,  Trenwick is required to deliver to the
Trustee an  Officers'  Certificate  and an Opinion of Counsel  stating  that the
Asset  Transfer and  supplemental  indenture in respect  thereto comply with the
provisions described in Articles IX and X of the Indenture;

         WHEREAS, Section 10.02 of the Indenture provides in pertinent part that
upon the Asset Transfer,  the successor Person to which Trenwick transferred its
property substantially as an entirety, shall succeed to, and be substituted for,
and may exercise  every right and power of Trenwick under the Indenture with the
same effect as if such successor Person had been named as Trenwick therein;

         WHEREAS,  Section 9.01 of the Indenture provides in pertinent part that
Trenwick and the Trustee may supplement the Indenture without the consent of any
Securityholder to provide for the assumption of Trenwick's obligations under the
Indenture to the Securityholders pursuant to Article X of the Indenture;


                                       1


         WHEREAS,  all things  necessary to make this  Supplemental  Indenture a
valid and binding  agreement of Trenwick and Trenwick America in accordance with
its terms have been done;

         NOW,  THEREFORE,  for  and  in  consideration  of the  premises,  it is
mutually  covenanted and agreed for the equal and  proportionate  benefit of all
Securityholders as follows:

         SECTION  1.  Trenwick  America  hereby  expressly  assumes  all  of the
obligations of Trenwick  under the  Indenture,  including the obligation to make
due and punctual payment of the principal of (and premium,  if any) and interest
on the Securities  according to their tenor and the due and punctual performance
and  observance of all the  covenants and  conditions to be kept or performed by
the Company under the Indenture.

         SECTION 2. Pursuant to Section 10.02 of the Indenture, Trenwick America
hereby  succeeds to, and is  substituted  for, and may exercise  every right and
power of  Trenwick  under the  Indenture  with the same  effect  as if  Trenwick
America had been named as Trenwick therein.

         SECTION  3.  After the  execution  and  delivery  of this  Supplemental
Indenture, any Securities authenticated and delivered in substitution for, or in
lieu of,  Securities then outstanding and all Securities  presented or delivered
to the Trustee on and after such date for such purpose  shall be stamped,  typed
or otherwise affixed with a notation as follows:

                  TRENWICK  AMERICA  CORPORATION,  A DELAWARE  CORPORATION,  HAS
                  EXPRESSLY  ASSUMED ALL OF THE  OBLIGATIONS  OF TRENWICK  GROUP
                  INC. UNDER THE INDENTURE, INCLUDING THE OBLIGATION TO MAKE DUE
                  AND PUNCTUAL PAYMENT OF THE PRINCIPAL OF (AND PREMIUM, IF ANY)
                  AND  INTEREST ON THE  SECURITIES  ACCORDING TO THEIR TENOR AND
                  THE DUE AND PUNCTUAL  PERFORMANCE  AND  OBSERVANCE  OF ALL THE
                  COVENANTS  AND  CONDITIONS  TO BE  KEPT  OR  PERFORMED  BY THE
                  COMPANY UNDER THE  INDENTURE.  REFERENCE IS HEREBY MADE TO THE
                  FIRST SUPPLEMENTAL INDENTURE,  DATED AS OF SEPTEMBER 27, 2000,
                  COPIES OF WHICH ARE ON FILE WITH THE TRUSTEE.

         SECTION 4. The Trustee accepts this  Supplemental  Indenture and agrees
to execute the trust  created by the Indenture as hereby  supplemented  upon the
terms  and  conditions  set  forth in the  Indenture,  including  the  terms and
provisions  defining and limiting the  liabilities and  responsibilities  in the
performance of the trust created by the Indenture as hereby supplemented.

                                       2


         SECTION 5. The Indenture,  supplemented as hereinabove set forth, is in
all  respects  ratified and  confirmed,  and the terms and  conditions  thereof,
supplemented  as  hereinabove  set forth,  shall be and remain in full force and
effect.

         SECTION  6.  The  validity  and  sufficiency,  as well as the  recitals
contained in this  Supplemental  Indenture  and the  statements in the Officers'
Certificate  and Opinion of Counsel  delivered in connection  herewith  shall be
taken as statements of Trenwick America and Trenwick, and the Trustee shall have
no responsibility for their correctness, validity or sufficiency.

         SECTION 7. This Supplemental Indenture shall become effective upon, and
simultaneously with, the effective time of the Asset Transfer.

         SECTION 8. The internal laws of the State of New York shall govern this
Supplemental Indenture.

         SECTION 9. This  Supplemental  Indenture may be signed in any number of
counterparts, each of which shall be an original, with the same effect as if the
signatures thereto and hereto were upon the same instrument.

         SECTION 10. Capitalized terms not otherwise defined herein are defined
as set forth in the Indenture.

         IN WITNESS  WHEREOF,  the parties hereto have caused this  Supplemental
Indenture to be duly executed as of the date first above written.

                                    TRENWICK AMERICA CORPORATION


                                    By: /s/ James F. Billett, Jr.
                                        ----------------------------------
                                        Name:  James F. Billett, Jr.
                                        Title: Chairman, President and Chief
                                               Executive Officer

                                    TRENWICK GROUP INC.


                                    By: /s/ James F. Billett, Jr.
                                        ----------------------------------
                                        Name:  James F. Billett, Jr.
                                        Title: Chairman, President and Chief
                                               Executive Officer

                                    THE CHASE MANHATTAN BANK, as Trustee


                                    By: /s/ Richard Lorenzen
                                        ----------------------------------
                                        Name:  Richard Lorenzen
                                        Title: Assistant Vice President



                                       3