D3.—(1) A person—
(a)who has paid additional contributions for a past period in accordance with Part II or Part IV of Schedule 4, or
(b)who has completed payment of additional contributions in accordance with Part I or Part III of Schedule 4, or with Schedule 5,
is entitled to count the past period as reckonable service.
(2) A person who has begun to pay additional contributions in accordance with Part I or Part III of Schedule 4, or has continued to pay them in accordance with Schedule 5, but who has not completed payment is entitled to count as reckonable service a period calculated in accordance with Schedule 7.