Went to the physio!
Dodgy leg is a combination of tight upper hamstring (hence pain in the bum) and sore hip flexor. Suspect the hamstring caused the hip flexor. So I have had multiple prods and massagey bits, and some ultrasound.
(If you scroll down to see a bum without skin on this page I think it is the semimembranosus)
Most of the time spent was trying to find a stretch that would work, I am apparently almost* hyper-mobile so normal stretches just don’t do anything for me. The tennis ball is a good thing, but I still don’t have one and am using a ball of wool today.
Having looked up hyper-mobile, I am indeed ‘almost’; I am flexible to the point of having difficulty stretching; but I don’t suffer from dislocations or any of the other nasty things. I am just relatively good at yoga without practice, and can annoyingly get my hands flat on the floor when trying to touch my toes (unless I have been on a long run…:) )
Really interesting looking site I found while poking around for hamstring stretches, but it seems the sun salutations, if done slowly, will be fine for the hip flexors.
Shall I buy a tennis ball? Decisions, decisions.