Capital gains tax shall be charged on the total amount of chargeable gains accruing to the person chargeable in the year of assessment, after deducting -
(a) any allowable losses accruing to that person in that year of assessment, and
(b) in so far as they have not been allowed as a deduction from chargeable gains accruing in any previous year of assessment, any allowable losses accruing to that person in any previous year of assessment (not earlier than the year 1974-75).