“(4A) Where in the case of a repayment the return on the basis of which the person's self-assessment was made under section 9 of this Act is enquired into by an officer of the Board— (a) nothing in subsection (3) or (4) above shall require the repayment to be made before the day on which, by virtue of section 28A(1) of this Act, the enquiry is completed; but (b) the officer may at any time before that day make the repayment, on a provisional basis, to such extent as he thinks fit”