Oxfordshire Sport Flying at Enstone do a lot of this sort of conversion training - PPL(SEP) to TMG, NPPL(SSEA) to SLMG, glider pilot to SLMG etc. They have a local TMG examiner and a rather nice Dimona motor glider with a tricycle undercarriage . Tel: 01608 677208.
Typical training time is around 5 hours, less if you are used to air brakes and wobbly prop.