I should update this thread.
I decided to take myself off to see a chiropractor. Best decision I could have made.
Turns out that on that mountainside walk on my honeymoon 30+ years ago, I damaged my hamstring attachment, and it never healed properly. Almost every issue I have had since, in hips, knees, ankles, balance, back and shoulders (and I have had all!) can be laid at the door of that original, neglected injury.
So I am seeing the chiro fortnightly and doing exercises set by her, firstly to undo the badly healed stuff, then to correct the consequences of it and finally to rebuild strength throughout.
It will be worse before it is better, but it will eventually be so much better! She has found no arthritis at all in any major joints, bar one tiny bit starting on the outside of the other hip (which now that we are aware of it, we can keep it at bay for as long as possible.) I can already see a lot of improvement in my balance, now that I am becoming less uneven, and am pretty sure that I will eventually not need the cocodamol I have been taking daily for about a decade - and stopping that will, says the chiro, give me more strength and stamina too.
So I would not wish my New Year's Eve ride on anyone, but it is going to turn out to be a blessing in disguise.