(1) In this section -
'excepted trade' has the same meaning as in section 21A;
'residential development' and 'residential development land' have the same meaning as each has in section 644A.
(a) Where in an accounting period a company carries on an excepted trade the operations or activities of which consist of or include dealing in land which, at the time at which it is disposed of by the company, is residential development land, the corporation tax payable by the company for the accounting period, in so far as it is referable to trading income from dealing in residential development land, shall be reduced by one-fifth.
(b) For the purposes of paragraph (a) -
(i) the corporation tax payable by a company for an accounting period which is referable to trading income from dealing in residential development land shall be such amount as bears to the amount of corporation tax for the period referable to income of an excepted trade the same proportion as -
(I) the amount receivable by th
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