4(1)In the computation of the gain accruing on the part disposal of a lease which is a wasting asset by way of the grant of a sublease for a premium the expenditure attributable to the lease under paragraphs (a) and (b) of section 38(1) shall be apportioned in accordance with this paragraph, and section 42 shall not apply.
(2)Out of each item of the expenditure attributable to the lease under paragraphs (a) and (b) of section 38(1) there shall be apportioned to what is disposed of—
(a)if the amount of the premium is not less than what would be obtainable by way of premium for the said sublease if the rent payable under that sublease were the same as the rent payable under the lease, the fraction which, under paragraph 1(3) of this Schedule, is to be written off over the period which is the duration of the sublease, and
(b)if the amount of the premium is less than the said amount so obtainable, the said fraction multiplied by a fraction equal to the amount of the said premium divided by the said amount so obtainable.
(3)If the sublease is a sublease of part only of the land comprised in the lease this paragraph shall apply only in relation to a proportion of the expenditure attributable to the lease under paragraphs (a) and (b) of section 38(1) which is the same as the proportion which the value of the land comprised in the sublease bears to the value of that and the other land comprised in the lease; and the remainder of that expenditure shall be apportioned to what remains undisposed of.