Your SEP allowed you to fly microlights.
So, you can either revalidate your SEP, or you can apply for an NPPL (microlights).
For an NPPL (M) you have to do this:
4.2 Pilots with expired licences or ratings
Credit shall be given for holders of expired CAA-issued JAR-FCL Pilot Licence (Aeroplanes) and UK
PPL(A) licences or ratings as follows:
a. Where a Microlight class rating or Microlight privileges included in such licences has expired
by not more than 5 years, the licence holder shall hold a valid NPPL medical declaration or
JAA Class 1 or 2 medical certificate and pass the NPPL GST in a Microlight.
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 JAA Class 1
or 2 medical certificate and pass the NPPL 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.
This is from the Allowances tab on the home page of the NPPL web site:
NPPL
All fairly straightforward. Obviously, much more training recommended if you want to fly a flexwing rather than a C42!