BLW - look under 'Licence Allowances' at
www.nppl.uk.com . You'll find that, with a BGA glider pilot's licence, you'll need not less than 10 hrs dual instruction, the exams, the medical, the 2 skill tests and that's about it!
To go from NPPL (SEP) to JAR-FCL PPL after qualifying for the NPPL (SEP) via this route has yet to be resolved; I will raise the point at our next NPPLSC meeting.