2.—(1) The contribution period must be one of not less than one year, and begins on the first day of the month following that in which the election becomes irrevocable under regulation C3(13).
(2) The contribution period and the past period must be such that
A + B
does not exceed 15 per cent of his contributable salary for the time being, where–
A is the rate at which the additional contributions are payable, and
B is the rate at which he pays other contributions under Part C (except any treated for the purposes of regulation G2 as employer’s contributions) or towards the provision of a pension otherwise than under these Regulations.