Hi Mike,
I've logged around 70hrs P1 time in gliding and have just completed my NPPL(SLMG) and added an (SSEA) to it.
Having the gliding knowledge/skills definately helped me on the power side; the glide approaches were a doddle
.
From CAP804:
(d) Crediting. Applicants holding a pilot licence for another category of aircraft, with
the exception of balloons, shall be credited with 10 % of their total flight time as PIC on such aircraft up to a maximum of 10 hours. The amount of credit given shall in any case not include the requirements in (a)(2).
Gliding is a whole different scene from powered flying clubs. If you have the time I highly recommend giving it a go.