No deduction shall be allowable in a computation of the gain more than once from any sum or from more than one sum.

References in this Chapter to sums taken into account as receipts or as expenditure in computing profits or gains or losses for the purposes of income tax shall include references to sums which would be so taken into account but for the fact that any profits or gains of a trade, profession, employment or vocation are not chargeable to income tax or that losses are not allowable for those purposes.

In this Chapter references to income or profits charged or chargeable to tax include references to income or profits taxed or as the case may be taxable by deduction at source.

For the purposes of any computation of the gain any necessary apportionments shall be made of any consideration or of any expenditure and the method of apportionment adopted shall, subject to the express provisions of this Chapter, be F1... just and reasonable.

In this Chapter “capital allowance” and “renewals allowance” have the meanings given by subsections (4) and (5) of section 41.