Where for the purpose of computing the gain or loss accruing to a person from the acquisition and disposal of any part of the new holding it is necessary to apportion the cost of acquisition of any of the original shares between what is disposed of and what is retained, the apportionment shall be made by reference to market value at the date of the disposal (with such adjustment of the market value of any part of the new holding as may be required to offset any liability attaching thereto but forming part of the cost to be apportioned).

This section has effect subject to section 81(2) below.