3.—(1) Subject to paragraph 4, where before entering the employment which he has ceased to hold the person had, on ceasing to hold a previous employment, became entitled to benefits—
(a)under regulation D7(1) or regulation E2(1)(b)(i) of the 1986 regulations, or
(b)under regulation 5(1)(a) of the Benefits regulations,
the additional period in relation to a person shall be a period calculated as mentioned in paragraph 2, but—
(i)on the assumption that his relevant membership period includes his relevant membership period (or, as the case may be, the period of his relevant service) in relation to the previous employment, and
(ii)with the deduction of the appropriate period.
(2) In sub-paragraph (1)(ii) “the appropriate period” means—
(a)in the case mentioned in sub-paragraph (1)(a), the additional period which became reckonable in relation to the previous employment by virtue of regulation D7(2) and this Schedule or, as the case may be, regulation E3(12) of the 1986 regulations, and
(b)in the case mentioned in sub-paragraph (1)(b)—
(i)if the person’s retirement pension in respect of the previous employment was calculated under regulation 5(3)(a) of the Benefits regulations, the period by which his relevant service fell short of 20 years, and
(ii)if that pension was calculated under regulation 5(3)(b) of those regulations, the period by which his relevant service would have been increased if he had continued in the previous employment until he attained the age of 65.