@vervain so, I was very interested to discover:

Experiment: I wedged myself in place, actually using a hard foam roller at my back. It felt quite comfortable.

Results: I still slept poorly, and woke up with my hip sore!

Conclusion: maybe I'm sleeping badly because I move around at night trying to alleviate pressure on my hip?

I'll look up occupational therapists, too, I'd forgotten about that. I spent a day phoning up physios and getting absolutely nowhere...