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                                                                    EXHIBIT 99.5
 
                              TRENWICK GROUP INC.
 
                           Offer for all Outstanding
                      6.70% Senior Notes due April 1, 2003
                                in Exchange for
                 6.70% Senior Exchange Notes due April 1, 2003,
                        which have been registered under
                     the Securities Act of 1933, as amended
 
To Our Clients:
 
     Enclosed for your consideration is a prospectus dated             , 1998
(the "Prospectus"), and the related letter of transmittal (the "Letter of
Transmittal"), relating to the offer (the "Exchange Offer") of Trenwick Group
Inc. (the "Issuer") to exchange its 6.70% Senior Exchange Notes due April 1,
2003, which have been registered under the Securities Act of 1933, as amended
(the "Exchange Notes"), for its outstanding 6.70% Senior Notes due April 1, 2003
(the "Old Notes"), upon the terms and subject to the conditions described in the
Prospectus and the Letter of Transmittal. The Exchange Offer is being made in
order to satisfy certain obligations of the Issuer contained in the registration
rights agreement in respect of the Old Notes, dated as of March 27, 1998,
between the Issuer and the initial purchaser referred to therein.
 
     This material is being forwarded to you as the beneficial owner of the Old
Notes held by us for your account but not registered in your name. A TENDER OF
SUCH OLD NOTES MAY ONLY BE MADE BY US AS THE HOLDER OF RECORD AND PURSUANT TO
YOUR INSTRUCTIONS.
 
     Accordingly, we request instructions as to whether you wish us to tender on
your behalf the Old Notes held by us for your account, pursuant to the terms and
conditions set forth in the enclosed Prospectus and Letter of Transmittal.
 
     Your instructions should be forwarded to us as promptly as possible in
order to permit us to tender the Old Notes on your behalf in accordance with the
provisions of the Exchange Offer. The Exchange Offer will expire at 5:00 p.m.,
New York City time, on             , 1998 unless extended by the Issuer. Any Old
Notes tendered pursuant to the Exchange Offer may be withdrawn at any time
before the Expiration Date.
 
     Your attention is directed to the following:
 
          1.  The Exchange Offer is for any and all Old Notes.
 
          2.  The Exchange Offer is subject to certain conditions set forth in
     the Prospectus in the section captioned "The Exchange Offer -- Conditions
     to the Exchange Offer."
 
          3.  Subject to the Letter of Transmittal, any transfer taxes incident
     to the transfer of Old Notes from the holder to the Issuer will be paid by
     the Issuer.
 
          4.  The Exchange Offer expires at 5:00 p.m., New York City time, on
                 1998, unless extended by the Issuer.
 
     If you wish to have us tender your Old Notes, please so instruct us by
completing, executing and returning to us the instruction form on the back of
this letter. THE LETTER OF TRANSMITTAL IS FURNISHED TO YOU FOR INFORMATION ONLY
AND MAY NOT BE USED DIRECTLY BY YOU TO TENDER OLD NOTES.
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                INSTRUCTIONS WITH RESPECT TO THE EXCHANGE OFFER
 
     The undersigned acknowledge(s) receipt of your letter and the enclosed
material referred to therein relating to the Exchange Offer made by Trenwick
Group Inc. with respect to its Old Notes.
 
     This will instruct you to tender the Old Notes held by you for the account
of the undersigned, upon and subject to the terms and conditions set forth in
the Prospectus and the related Letter of Transmittal.
 
     Please tender the Old Notes held by you for my account as indicated below:
 

                                                            
                                                               AGGREGATE PRINCIPAL AMOUNT OF OLD NOTES
6.70% Old Senior Notes due April 1, 2003
 
[ ] Please do not tender any Old Notes held by you for
    my account.
 
Dated: ____________________, 1998                              --------------------------------------------------------
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                                                               SIGNATURE(S)
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                                                               PLEASE PRINT NAME(S) HERE
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                                                               ADDRESS(ES)
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                                                               AREA CODE AND TELEPHONE NUMBER
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                                                               TAX IDENTIFICATION OR SOCIAL SECURITY NO(S).

 
     None of the Old Notes held by us for your account will be tendered unless
we receive written instructions from you to do so. Unless a specific contrary
instruction is given in the space provided, your signature(s) hereon shall
constitute an instruction to us to tender all the Old Notes held by us for your
account.