18.—(1) Subject to paragraph 21, a person who was notified before becoming entitled to payment of retirement benefits may, by giving written notice to the Secretary of State within 3 months after the relevant date, elect that his war service period is to be included in his family benefit service.
(2) The relevant date—
(a)if he applied for payment of retirement benefits before becoming entitled to payment of them and before being notified, is the date on which he was notified, and
(b)in any other case, is the date on which he applied for payment of retirement benefits.