I spent a great deal of unpaid time and trouble putting together the NPPL Cross-Credit document, so please see
http://www.nppl.uk.com/documents/NPPLXCREV08_000.pdf rather than pestering the BMAA or CAA with questions which you can probably answer for yourselves.
For example:
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
ALL SEP Class Ratings include Microlight privileges; the CAA, however, require that people conduct some training before exercising them.
So the person with a lapsed SEP Class Rating who now wants to fly Microlights but doesn't want to fly SEP Class aircraft again merely needs to obtain a NPPL Medical Declaration, pass the Microlight GST (which includes an oral examination), then apply for a NPPL with Microlight Class Rating.