Just to return to this, after a bit more research, my understanding of the NPPL route to a TMG rating is:
Grant of NPPL from CAA based on existing PPL £49
SLMG Flight training as required and test - say £350
SLMG rating added to NPPL £69
TMG Rating added to PPL £84
Is this correct? The CAA direct route seems more straightforward, though training appears to have to be on specific types (eg Super Dimona), which are more expensive so this balances out against the increased admin costs of the NPPL route, the only problem seems to be finding a CAA Instructor - does anyone know of any in the north of England?