If you're talking about a JAA PPL or full UK PPL then you can fly a TMG (an SLMG which does not have a retractable engine/prop) only once you have a TMG Class Rating. To get this you need to pass a skill test (stalls, steep turns, PFL, circuits) with a TMG Class Rating Examiner. In the UK only once you have a TMG class rating you can fly all SLMGs.
As far as an NPPL is concerned, if you have a full PPL for SEP aircraft you can automatically get issued with the equivalent NPPL. Then you just need differences training to be allowed to fly SLMG.