13.—(1) The contribution period begins on the commencement date, that is to say 1st October next following the date on which notice of the principal election was given.
(2) Where at the commencement date the person has attained the age of 55, the contribution period must be one of not less than one nor more than 5 years.
(3) In any other case, the contribution period must be one of not less than one nor more than 10 years, ending on or before his 60th birthday.
(4) The contribution period and the past period must be such that at the commencement date A+B does not exceed 15% of his contributable salary, 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.