20 For the purposes of this Schedule, the “normal rent” in respect of a lease for a period of account of the lessor is the amount which he would, apart from this Schedule, bring into account as rent from the lease—
(a)which arises to him, and
(b)if rent arising to him from the lease is chargeable to corporation tax under Schedule A, to which he is entitled,
in that period of account for the purpose of determining his liability to tax for the related chargeable period or periods.