Where on a claim made for the purpose it is shown that the whole or any part of a chargeable transfer ("the relevant transfer") has by virtue of any enactment or rule of law been set aside as voidable or otherwise defeasible—
tax paid or payable by the claimant (in respect of the relevant transfer or any other chargeable transfer made before the claim) that would not have been payable if the relevant transfer had been void ab initio shall be repaid to him by the Board, or as the case may be shall not be payable, and
the rate or rates of tax applicable to any chargeable transfer made after the claim by the person who made the relevant transfer shall be determined as if that transfer or that part of it had been void as aforesaid.
In subsection (1)(a) above the reference to tax includes a reference to interest on tax.
A claim under this section must be made not more than 4 years after the claimant knew, or ought reasonably to have known, that the relevant transfer has been set aside.