8(1)The Commissioners are not liable, on any claim for a repayment of plastic packaging tax, to repay any amount—
(a)paid more than 4 years before the making of the claim;
(b)if, or to the extent that, any person has become entitled to a tax credit in respect of that amount.
(2)It is a defence to any claim for repayment of an amount of plastic packaging tax that the repayment of that amount would unjustly enrich the claimant.