Hi Ellie,
I do both so hopefully you will enjoy gliding! Each flight is unique and the challenge never dies! I'm not in the same league as the legendary Mary but I hope my answers below will suffice in the meantime!
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: My own personal view is that power PFL and glider field landings are apples and oranges; different initatiing reasons, techniques and workloads. Gliding will probably polish your field selection skills and make you more comfortable with the concept of landing out.
2) Should I 'leave' my flying experience to date behind, (& start from scratch with no pressure), or fess up my background to the instructor: Definitely admit to it - you'll get paired with the correctly rated instructor and you can just get on with the differences training. The majority of my gliding colleagues fly power too.
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: Learning generally cheaper - the initial trial lesson tends to include a bung towards insurance. Expect £20-£30 for a tow to 2000' / £7-£10 for a winch launch to 1200'. Annual memberships varies from £200 to £500. Budget a grand and you'll be bomb proof.
4) Are there any recommended books to read on the subject of gliding: Understanding Gliding by Derek Piggot is the classic text and still worth a read. The BGA website has some good suggestions.
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..! It is pretty rare to end up in the hedge and is normally the result of pulling the wrong lever or not putting the right lever away. Gliders have stunning airbrakes and respond well to sideslipping in an overshoot scenario. In an undershoot scenario closing the brakes has a dramatic effect in lift restoration. The key skill is anticipating windshear but it's all pretty straightforward.
Where abouts are you based? Most clubs are pretty good.
Go for it!