The available surplus in respect of any accounting year shall be the gross profits for that year after deducting therefrom the sums referred to in section 6:

Provided that the available surplus in respect of the accounting year commencing on any day in the year 1968 and in respect of every subsequent accounting year shall be the aggregate of—

(a) the gross profits for that accounting year after deducting therefrom the sums referred to in section 6; and

(b) an amount equal to the difference between—