54(1)This paragraph applies where—
(a)an amount has been paid by way of a repayment of tax, and
(b)the amount paid exceeded the amount which the Commissioners were or could be liable at that time to repay.
(2)The Commissioners may—
(a)to the best of their judgment, assess the amount of the excess, and
(b)notify the amount to the person to whom the repayment was made.
(3)Sub-paragraph (4) applies where—
(a)an assessment has been notified under sub-paragraph (2), and
(b)it appears to the Commissioners that the amount which ought to have been assessed as due exceeds the amount that has already been assessed.
(4)The Commissioners may—
(a)on or before the last day on which the assessment could have been made, make a supplementary assessment of the amount of tax due, and
(b)notify the amount to the person to whom the repayment was made.
(5)An amount assessed and notified under sub-paragraph (2) or (4) counts as a liability to multinational top-up tax for the purposes of this Part of this Act.
(6)But sub-paragraph (5) does not have effect if, or to the extent that, the assessment has been withdrawn or reduced.
(7)An assessment under this paragraph may not be made more than 4 years after the end of the accounting period in which evidence of facts sufficient in the opinion of the Commissioners to justify making the assessment comes to their knowledge.