Thanks Whopity, but perhaps I wasn't too clear. I intend obtaining NPPL SSEA on the basis of my PPL(A), then adding SLMG through differences. My source was a well known motorgliding school website which says this:
NPPL SSEA to SLMG
This is an easy conversion, and all you need to do is complete ‘differences training’ and then get signed off (no skills test required!)