DFC is quite right. In a quirk of the ANO (see sched.8) it states (PPL privileges):
(b) He shall not receive any remuneration for his services as a pilot on a flight save
that if his licence includes a flying instructor’s rating, a flight instructor rating or
an assistant flying instructor’s rating by virtue of which he is entitled to give
instruction in flying microlight aircraft or self-launching motor gliders he may
receive remuneration for the giving of such instruction or the conducting of such
flying tests as are specified in sub-paragraph (a)(i) in a microlight aircraft or a selflaunching
motor glider.
So you could be a paid PPL/FI giving instruction on a Grob (TMG) but couldn't be paid for converting the post-qualified student to a Cessna 150 (time required ~1 hr). Logic?
Incidentally the TMG training could also be done at an unlicensed field.