I lost my SLMG when I went from a UK PPL to a French one (It is a natioal rating) In France you can fly any motor glider on a glider pilot's licence, with an instructor sign-off for each different type. This may change in the future, so I have just done my TMG on a Dimona. Had to be with the right sort of examiner, i.e. Power and Glider. It allows me to fly TMG but not retractable engine motor gliders, that still requires a glider pilot's licence. I may stop the engine in flight but only while I continue to hold a valid glider pilot's licence. It does mean that I now have an international licence for TMG. Confused? Good. So am I.
Cost? Two hours flying, an hour with an instructor for type familiarisation, followed by an hours test. That gives me the TMG with tailwheel and VP variants. The Dimona is a pretty easy taildragger, as taildraggers go. The prop is a manual three position one, fine, coarse, and feathered.
The DGAC don't charge to put an extra rating on an existing licence. i'll be driving to Poitiers this week to the local DGAC office to get the stamp in my liçence.