Thanks for all the ideas and advice, everyone.
Yes I had my hip assessed, including x-rays, the first time it did this - which was after a many miles long walk alongside the slope of a mountain in Scotland, on my honeymoon! So I was only early 30s at the time. X-rays showed nothing, but I was in agony, literally, for some time before it settled. It's been on and off grumbling pretty much ever since. But the "off" can be months or even years, to the point where I can't even remember which is the dodgy hip. Then it has a grumble, and I remember :/.
I have also had x-rays of my one big toe, at that moment the worst spot for pain (after using it to take all my weight as I climbed over hurdles literally hundreds of times in a short period, during lambing), about 15 years ago (so 50 ish by then). Again, nothing to see, but the doc said he'd put money on osteoarthritis nonetheless.
Further GP checks on all leg joints a few years later (no x-rays, just a physical exam), he said nowhere near bad enough to consider taking it further for now. Looking back, I realise I just allowed my physical activity to decrease to avoid pain, even though it meant giving up some activities I really enjoyed - of which one was riding my (very broad) Fell ponies.
Increasing levels of discomfort, especially in winter, was a big factor in my decision to move back to the SW of England. If I had stayed in Cumbria, I would have had to increase the meds, and I preferred to defer that stage if possible.
This last 8 months or so, I have been doing yoga again, and I had a little go on Davy in the summer to see if my hips looked up to it, before deciding on whether to look for a rideable successor or just a fieldmate for Floss, (which could be a youngster to bring on to ready to ride, then sell, or a non-ridden pony.) No pain whatsoever, it being summer and the yoga being very helpful. I couldn't do a lot with Davy as his tumour made using any kind of bridle or halter potentially painful for him, but previously, when still in Cumbria, I had been unable to even sit astride for a moment without significant discomfort, so I figured I looked to be good for a bit more riding yet, and searched for something rideable.
I do find diet to be important, yes. Not GAPS exactly but a similar regime (Harcombe plan) works for me. (No refined sugar at all, aim for slow-release carbs, nothing processed, never be hungry, and avoid eating much in the way of fats at the same meal as carbs.) If I watch it, I can eat lovely food and lose weight - not without trying, exactly, but without misery! - if I want to. It's tough to manage with communal meals, though, so I have to avoid some of the meals (and almost all of the puds :/) that are offered, but it's a small price to pay for feeling healthier and happier.
Western saddle is a non-starter with Flo, unfortunately, as she is very short-backed (even more so than most Fells, and in fact I think few Fells have a long enough back for a western saddle), but the TCS gives you the same ability to sit in a more comfortable position and doesn't constrain your position in any way, so you can adjust your position and how you carry your weight as you need. On my hack, I was actually not in anything like as much pain if I kicked the left stirrup off while in walk, so I am pretty sure that being less constrained is a big factor. If I had felt a little more sure of the pony and my own balance, I would maybe have crossed the stirrups on his neck and ridden stirrupless the rest of the ride. But his trot was quite staccato, and not as comfortable to ride sitting as other horses, and I was worried I might bounce off, or make him uncomfortable. It was a 7-person ride - 5 clients and 2 staff - so I didn't want to impact the ride too much for others, as far as possible, or I would have done stirrupless walking, and get the stirrups back for trotting. Rising trot was much more comfortable than walking in stirrups, even so, which is another thing that makes me think being stuck in one position was a huge factor. It's maybe something the 20- and 30-something BHSAIs are not going to really be able to appreciate, but those of us who know about being stiff getting up out of a chair if we've sat a little too long...