Where in pursuance of the provisions of any will or settlement any income arising from any fund is accumulated for the benefit of any person contingently on his attaining some specified age or marrying, and the aggregate amount in any year of assessment of that income and the income from any other fund subject to the like trusts for accumulation and of the total income of that person (hereinafter referred to as “the aggregate yearly income”) is of such an amount only as would entitle an individual either to total exemption from tax or to relief from tax, that person shall, on making a claim for the purpose within six years after the end of the year of assessment in which the contingency happens, be entitled, . . . F1, to have repaid to him on account of the tax which has been paid in respect of the income during the period of accumulation a sum equal to the aggregate amount of relief to which he would have been entitled if his total income for each of the several years of the said period had been equal to the aggregate yearly income for that year; but in calculating that sum a deduction shall be made in respect of any relief already received.