“(4) Any sum paid by an employer by way of contribution under the scheme shall, for the purposes of Case I or II of Schedule D and of sections 75 and 76, be allowed to be deducted as an expense, or expense of management, incurred in the chargeable period in which the sum is paid … (5) The amount of an employer’s contributions which may be deducted under subsection (4) above shall not exceed the amount contributed by him under the scheme in respect of employees in a trade or undertaking in respect of the profits of which the employer is assessable to tax (that is to say, to United Kingdom income tax or corporation tax). (6) A sum not paid by way of ordinary annual contribution shall for the purposes of subsection (4) above be treated, as the Board may direct, either as an expense incurred in the chargeable period in which the sum is paid, or as an expense to be spread over such period of years as the Board think proper. …”