1) As well as learning generally, I'm interested in what gliding could bring to my PFL teaching skills. With this in mind, would a motor glider be a better option
Many clubs use motor gliders for cross country training but I recommend having a few trial flights in a regular glider to get the full experience. Perhaps aim for 2-3 winch launches and an aero tow to say 4000ft if it's not been a very good day for thermals.
2) Should I 'leave' my flying experience to date behind, (& start from scratch with no pressure), or fess up my background to the instructor
Tell him.
3). What sort of costs can I expect? On looking on gliding websites, trial lessons are almost as much as powered fixed wing ones are - is this the case for subsequent lessons
No. Some Clubs sell "trial lessons" because their insurance won't allow them to carry "passengers" or sell "air experience" flights. They bring in money for the club. Lessons as a Club member will be much cheaper. You do need to commit time. You need to fly regularly to remain current and will be expected to help run the club. That may mean learning how to be a launch marshal or learning how to run the winch and perhaps doing that on a rota.
4) Are there any recommended books to read on the subject of gliding
See previous recommendations.
5). Finally, (& excuse my ignorance on this one), what do glider pilots do if they have messed up the landing back at the gliding field/site, as they can't go around..... Surely it does happen sometimes..!
You make sure you don't mess up. They have quite good air brakes and the ideal approach is made with half brake to give you options in both directions. The only time I messed up was when I landed a bit long on wet grass, had to put a wing tip on the ground to make it turn to avoid a ditch.
I got hooked after visiting a club near Zurich. I just walked into a club and asked if I could have a go. They were great and called up an instructor that lived locally. Next day I made a 4 hour flight in the mountains. Soared with birds, used thermal, ridge and wave lift, went to 13,000ft, flew past climbers half way up a sheer rock face, flew over a glacier and generally blew my mind.