By way of explanation to anyone not following the acronyms:-
This may not be the legally correct answer but within gliding world an SLMG tends to be regarded as the aircraft that looks like a normal glider but has a pop up engine in the back and has sufficient power to self launch. With the engine stowed these a/c have the performance of a normal glider. (OK and there's the Stemme S10 which has a pop-out prop at the front)
A TMG is the one which looks more like a "normal" light aircraft e.g. engine & prop bolted to the front. The aircraft has longer wings and the prop can be feathered for gliding flight. They need a pretty good thermal to climb engine-off but have a much better glide angle than your average GA a/c.
Self sustainers look like a normal glider but have a pop up engine which does not (in theory anyway!) have enough power to take off unassisted but does (just about) have enough thrust to get you home in more ore less level flight when the thermals switch off.
From the glider pilot's perspective the important distinction between the SLMG/TMG and the self sustainer (SSMG) is that the SSMG can be flown by any glider pilot without needing all the PPL etc etc qualifications being discussed in this thread. Also (pre EASA) the SSMG's were covered by BGA CofA's and could be signed off by BGA inspectors - and hopefully will continue to be in the future.
The SLMG/TMG's have to conform to normal power aircraft maintenance rules (and costs) - but they don't need dual ignition