Well I did see the osteopath, she was fab - she either knew all about HMS anyway, or had done a ton of reading before I went to see her as she knew what to watch out for, what NOT to do, she was really good. She sat and talked with me for quarter of an hour before she examined me, then got me to bend 'comfortably' in different ways (she did go a bit

when I bent double and put my hands on the floor, she said "Oh that would be the hypermobility then"

).
She said my back was incredibly tight, that the muscles had all locked to stop my spine moving around. I had no movement at all in the top and bottom of my back, but the middle was doing all the bending and had twisted due to the strain. She did a lot of massage, some of it quite 'deep', and some gentle stretching; she did 'pop' the middle of my back to straighten it but it was very gentle, she didn't do any big crunching movements. She told me to go home and put ice packs on my lower back and glutes where she had done the massage, and she was right to do that - I felt like I'd been kicked! But I had my first night's sleep in AGES without painkillers, and it all feels much more comfortable now.
She didn't prescribe a 'course' of treatment, just said to come back when I felt like it - she recommended about 10 days, but said to leave it longer if I felt ok. I didn't feel under any pressure at all to make more appointments, which was really nice (although I've already booked for the end of next week, the massage was definitely worth it!).