There's no requirement to pass a flight test to get an NPPL SLMG rating.
Whether you go straight to TMG rating or through the NPPL SLMG, you will need a PPL, a valid medical & training to an adequate standard (around 2 hours ought to be enough).
Then,
either you hire a TMG and and pay an examiner to test you,
or you pay the NPPL chaps for an NPPL with SSEA & SLMG ratings (no test).
Once you've done either of the above, then you pay the CAA for the addition of a TMG rating to your PPL.
Section F, appendix D of LASORS defines what is a TMG/SLMG or SLMG only. Hus Bos'
Rotax Falke counts as both. I've never heard of anyone getting an NPPL SLMG in a Nimbus or DG500 but I suppose it's possible.
I think that the cheque to NPLG (payable to the CAA) includes the £49 for the grant of an NPPL (check with the NPLG). Wouldn't there also be a CAA £192 fee for the type rating test if you do a TMG flight test?
(Adam - Touring Motor Glider)