<PAGE>

                                                                   Exhibit 10.12


      FORM OF AMENDMENT TO THE TA I LIMITED ZERO COST SHARE OPTION SCHEME

      Pursuant to rule 7.1 of the TA I Limited Zero Cost Share Option Scheme
      ("the Scheme"), the Board of Directors of TA I Limited, hereby amend the
      Scheme as follows:-

      1.    The definition of "the Company" shall be amended to read as follows:


            "THE COMPANY" means Willis Group Holdings Limited (a company
            registered in Bermuda);

      2.    In the definition of "Group Member" the reference to "section 736 of
            the Companies Act 1985" and to "section 258 of that Act" shall be
            deleted and replaced by the words "section 86 of the Bermudan
            Companies Act 1981".


      3.    In the definition of "Subsidiary" the reference to "section 736 of
            the Companies Act 1985" shall be deleted and replaced by the words
            "section 86 of the Bermudan Companies Act 1981".


      4.    In Rule 5.3, the reference to section 428 to 430F of the Companies
            Act 1985 shall be deleted and the words "section 102 of the Bermudan
            Companies Act 1981" shall be inserted and the reference to section
            425 of the Companies Act 1985 shall be deleted and the words
            "section 99 of the Bermudan Companies Act 1981" shall be inserted.

      5.    A new Rule 5.4 shall be inserted to read as follows:

            "If any company ("the acquiring company"):

            5.4.1 obtains control of the Company as a result of making:

            (a)   a general offer to acquire the whole of the issued ordinary
                  share capital of the Company which is made on a condition such
                  that if it is satisfied the person making the offer will have
                  control of the Company, or


<PAGE>

            (b)   a general offer to acquire all the shares in the Company which
                  are of the same class as the shares which may be acquired by
                  the exercise of options granted under this Scheme, or

            5.4.2 obtains control of the Company in pursuance of a compromise or
                  arrangement sanctioned by the Court under section 99 of the
                  Bermudan Companies Act 1981, or

            5.4.3 becomes bound or entitled to acquire shares in the Company
                  under section 102 of the Bermudan Companies Act 1981,

            any Participant may at any time within the appropriate period, by
            agreement with the acquiring company, release any option which has
            not lapsed ("the old option") in consideration of the grant to him
            of an option ("the new option") which is equivalent to the old
            option but relates to shares in a different company."

            6.    A new Rule 5.5 shall be inserted to read as follows:

            "Where a new option is granted, the provisions of this Scheme shall
            for this purpose be construed as if:

            5.5.1 the new option were an option granted under this Scheme at the
                  same time as the old option;

            5.5.2 the expression "the Company" were defined as "a company whose
                  shares may be acquired by the exercise of options granted
                  under this Scheme."