3.—(1) This paragraph applies where a teacher–
(a)has elected to pay additional contributions to purchase past added years under regulation C5; or
(b)has continued to pay additional contributions to purchase past added years by virtue of regulation C7(1),
and before attaining the age of 60 and before the end of the period during which they were to be paid he or she ceases to be in pensionable employment, otherwise than by reason of his or her death or his or her becoming incapacitated or circumstances rendering him or her entitled to retirement benefits by virtue of regulation E6(1)(d).
(2) Where this paragraph applies, any right to elect to make a lump sum payment may be exercised only to the extent that it does not result in the addition to the service that the teacher would otherwise have been entitled to count as reckonable service of more than–
where–
A is the longest period in respect of which he or she could have elected to pay the additional contributions;
B is the length of his or her reckonable service, excluding any past period reckonable by virtue of additional contributions, when he or she ceased to be in pensionable employment;
C is the total of B and the period beginning at the cessation and ending immediately before his or her 60th birthday; and
D is the length of the past period or periods reckonable at the time of the cessation by virtue of all additional contributions.