You require to pass the TMG Class rating LST with an examiner holding a TMG Examiners rating and then pay the CAA the relevant fee to have the rating entered into your licence.
If you haven't flown TMG before then you will probably require several hours with a TMG instructor as these aircraft require different handling during approach and landing e.g. spoilers/airbrakes instead of flaps.
To my knowledge all motor gliders that fall within the relevant definition require either a TMG or SLMG rating to fly them legitimately. (the situation may be different for service aircraft)
Motor gliders are seperated into SLMG and TMG the former being sailplanes with retractable powerplants and the latter fixed power plants. Within the UK FIR you can fly both groups on either an SLMG or TMG rating. Self sustaining sailplanes i.e. TURBO's do not currently require a pilots licence.
With the TMG rating the hours flown in TMG and SEP are mutually acceptable for the revalidation of both ratings. With an SLMG rating the hours flown in a motor glider only count for their SLMG revalidation while the SEP hours only count towards the SEP
revalidation requirements. The latter is relevant to the NPPL only as you cannot obtain an SLMG licence within the UK on a JAR licence.
In your case the Bronze C makes no difference other than that you will obviously have experience of operating with spoilers/airbrakes.
The dispensation now available is for initial issue of the rating only and permits certain BGA member clubs to offer courses to their members for the gaining of an NPPL SLMG. The holding of a Glider Pilots Licence reduces the flying requirements to 10 hours.
Trust that this clarifies the matter.