19.—(1) Subject to paragraph (2) and regulation 21 a licence shall remain in force until –
(a)the expiration of a period of six months commencing on the date of issue, or
(b)the day immediately following the day on which the holder of the licence failed the instructional ability and fitness test at the third attempt, whichever shall first occur.
(2) Where a person applies for a new licence in substitution for a licence held by him and current at the date of the application, the previous licence shall not expire until –
(a)the commencement of the new licence, or
(b)if the Registrar decides to refuse the application, until the time limited for an appeal under regulation 24 against the decision has expired and, if such an appeal is duly brought, it is finally disposed of.