I did my NPPL a few years ago and having contacted the BGA I sent them my Glider Pilot certificate which included Silver C and they sent me a Glider Pilots License, which then allowed me to complete the NPPL in some 22 hours,the minimum being 13 hours.
As an aside I recommend doing the 150 Nautical mile QXC as the 100 needed for the NPPL is neither use nor ornament for any thing else.(Not ICAO compliant) If you want to fly in France on an NPPL you need the 150nm flight. You also need it if you upgrade to a JAA PPL.
Hope this helps.