18(1)An assessment of a penalty under this Schedule in respect of any amount must be made on or before the later of date A and (where it applies) date B.
(2)Date A is the last day of the period of 2 years beginning with the date specified in or for the purposes of column 3 of the relevant table in paragraph 1 (that is to say, the last date on which payment may be made without incurring a penalty).
(3)Date B is the last day of the period of 12 months beginning with—
(a)the end of the appeal period for the assessment of the amount of tax in respect of which the penalty is assessed, or
(b)if there is no such assessment, the date on which that amount of tax is ascertained.
(4)In sub-paragraph (3)(a) “appeal period” means the period during which—
(a)an appeal could be brought (ignoring any possibility of an appeal out of time), or
(b)an appeal that has been brought has not been determined or withdrawn.