You can add a Microlight Class rating to a UK lifetime PPL. Since you have an expired SEP class rating, see CAP 804, Section 5, Appendix 1, page 19:
SECTION 4 CROSS-CREDITING LICENCES AND RATINGS TO NPPL (MICROLIGHT)
4.2 Pilots with expired licences or ratings
Credit shall be given for holders of expired CAA-issued Part-FCL Pilot Licence (Aeroplanes) and UK PPL(A) licences or ratings as follows:
a) ...
b) Where a Microlight class rating or Microlight privileges included in such licences has expired by more than 5 years, the licence holder shall undergo a course of training in a Microlight aircraft as specified by a Microlight FI, hold a valid NPPL Medical Declaration or Part-MED Class 1, 2 or LAPL medical certificate and pass the NPPL(A) GST in a Microlight. The licence holder shall also pass an oral theoretical knowledge examination conducted by the authorised examiner as part of the GST.
No written examination required, just training as required, pass the GST, and apply for a Microlight class rating.
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