You are using cross-crediting to obtain the SLMG, so need to produce evidence of a valid SSEA rating. See page 9:
http://www.nationalprivatepilotslice...20REV%2010.pdf
The holder of a valid NPPL with SSEA Class Rating, UK PPL (A) or CAA-issued JAR-FCL Pilot Licence (Aeroplanes) with SEP Class Rating who wishes to obtain a SLMG Class Rating shall :
a. Produce the NPPL (SSEA), UK PPL (A) or CAA-issued JAR-FCL Pilot Licence (Aeroplanes);
b. Produce log book evidence of having satisfactorily completed conversion training with a SLMG Instructor on self-launching motor gliders;
c. Hold a valid NPPL Medical Declaration or Part-MED LAPL, Class 1 or 2 medical certificate.
On the application form:
http://www.nationalprivatepilotslice...20Rev%2017.pdf
you are requested to send:
Copy of any existing Licences / Certificates of revalidation/renewal for which credit is sought
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