If I'm correct, TMG is under PPL and SLMG is under NPPL (even though they're the same craft!!). I don't have a NPPL so can't log Dual SLMG (what bureaucratic nonsense!!) I guess to log the hours I need to get the TMG class rating on my PPL and otherwise I will have no record (other than specifically for gliding).
Not quite. The old CAA and (I think) early JAR PPL's had an SLMG rating attached, if not you can add it I think (though you'd be better to just go straight to a TMG rating). You can fly SLMG (and log the hours) on these licences, but if you want to use the hours to maintain currency then you need to upgrade the SLMG rating to TMG. Like you say, the aircraft are the same, just the name of the rating differs.
SS