You can go from NPPL [SLMG] to NPPL [SSEA] by completing differences training and sending the form and dosh off to NPLG at Turweston. Quite a simple process.
Details here:
http://www.nationalprivatepilotslice...20REV%2008.pdf
NPPL (SLMG) or UK PPL (A) SLMG to NPPL (SSEA)
The holder of a valid NPPL with SLMG Class Rating or UK PPL (A) SLMG who wishes to obtain a SSEA Class Rating shall:
a. Produce the NPPL or UK PPL (A) SLMG;
b. Produce log book evidence of having satisfactorily completed SSEA conversion training with a FI(A) or CRI(SPA) on single-engine piston aeroplanes;
c. Hold a valid NPPL Medical Declaration or JAA Class 1 or 2 medical certificate.
You can then exercise NPPL [SSEA] privileges with all of the hours that you fly counting towards JAR-FCL issue. Make sure that you fulfil the solo and navigation requirements. Then look at LASORS 6.2 for the requirements to upgrade to a JAR-FCL PPL with SEP class rating.
http://www.caa.co.uk/docs/175/Sectio...%20LICENCE.pdf
The dual hours that you use to convert from SLMG to SSEA count, as all NPPL [SSEA] instructors are currently JAR-FCL instructors also.
ifitaint...