Hi Lotzy,
One thing we have given our cat, as shes getting on a bit and was getting a bit 'creaky' is a self-heating mat for her bed?
Its a wooly 'pillowcase' type affair, with a reflective material layer inside it, kind of like that foil stuff they wrap athletes in, but not so crinkly LOL!
It does rustle a bit, so it might not be to your cats liking, but ours really loves it. Her bed is by the radiator, and when shes asleep on it her body heat 'reflects' back up at her and keeps her extra toasty, so her joints stay nice and warm. We noticed that the 'creaking' lessened quite a bit after she started using it.
I think the one we got was an Ancol one, and it was the small size I think.
Might not be as effective for more serious arthritis, but it certainly might help make things a bit more comfy?
H