The Principal Act is amended by inserting the following section after section 21 -
(1) For the purposes of this section -
'claimant' means a person who submits a valid claim for a refundable amount;
'overpaid amount' means an amount which is a refundable amount as a result of a claimant having made a payment of tax;
'refundable amount' means an amount which a person is entitled to receive from the Revenue Commissioners in accordance with this Act or any order or regulation made under this Act and which is claimed within the period provided for in subsection 20(4), but such amount does not include interest payable under this section; 'valid claim' means a return or a claim, furnished in accordance with this Act or any order or regulation made under it, and which includes all information required by the Revenue Commissioners to establish the refundable amount.
(2) Where a mistaken assumption in the operation of the tax is made by the Revenue Commissioners and as a result a ref