13.—(1) In this regulation—
“overdeclaration” means, in relation to any return, the amount (if any) which was wrongly treated as tax due for the accounting period concerned and which caused the amount of tax which was payable to be overstated, or the entitlement to a payment under regulation 20 to be understated (or both) or would have caused such an overstatement or understatement were it not for the existence of an underdeclaration in relation to that return;
“underdeclaration” means, in relation to any return, the aggregate of—
the amount (if any) of tax due for the accounting period concerned which was not taken into account; and
the amount (if any) which was wrongly deducted as credit,
and which caused the amount of tax which was payable to be understated, or the entitlement to a payment under regulation 20 to be overstated (or both) or would have caused such an understatement or overstatement were it not for the existence of an overdeclaration in relation to that return.
(2) This regulation applies where a registrable person has made a return which was inaccurate as the result of an overdeclaration or underdeclaration.
(3) Where in any accounting period a registrable person has discovered one or more overdeclarations, he may enter the overdeclarations in the return for the accounting period in which they were discovered by including their amount in the box opposite the legend “Overdeclarations from previous periods (no limit)”.
(4) Where in any accounting period—
(a)a registrable person discovers one or more underdeclarations; and
(b)having treated the amount of those underdeclarations as reduced by the amount of any overdeclarations for the same accounting periods, the total of those underdeclarations does not exceed £2,000,
he may enter the underdeclarations in his return for the accounting period in which they were discovered by including their amount in the box opposite the legend “Underdeclarations from previous periods (must not exceed £2,000, see general notes)”.
(5) Where a registrable person enters an amount in a return in accordance with paragraph (3) or (4) above he shall calculate the tax payable by him or the payment to which he is entitled accordingly.
(6) Where an amount has been entered in accordance with this regulation in a return which has been made—
(a)the return shall be regarded as correcting any earlier return to which that amount relates; and
(b)the registrable person shall be taken to have furnished information with respect to the inaccuracy in the prescribed form and manner for the purposes of paragraph 20 of Schedule 5 to the Act.
(7) No amount shall be entered in a return in respect of any overdeclaration or underdeclaration except in accordance with this regulation; and as regards any underdeclaration that cannot be corrected under paragraph (4) above a person shall not be taken to have furnished information with respect to an inaccuracy in the prescribed form and manner for the purposes of paragraph 20 of Schedule 5 to the Act unless he provides such information to the Commissioners in writing.