Well, the decision has been mulled over and finally made and I settled for a Baavet bed, because as
@Selina27 mentioned it was firm but not Memory Foam (which Joy previously refused to sleep on.)
I was torn between the medium and large sizes, eventually settling for medium, as the measurements were the same as Joy's current bed.
When it arrived I put it inside a clean but now redundant duvet cover incase I needed to return it, which is just as well, because I did! Although the measurements were the same as her old bed, the new one was much firmer ( which I'd wanted, due to her arthritis), so whilst it was fine while she was curled up, she wasn't comfortable when it came to sprawling out on her back, which is how she always ends up. This meant posting it back, which was a bit of a palaver and cost over £14.00! I was somewhat in two minds about ordering another, incase she didn't like that one either, but thought I'd give the larger size a try, as I'd really liked the supportive feel of the bed and thought it might help with Joy's arthritis. Well, I have to say that I've been impressed with how pleasant and helpful the people at Baavet have been during our telephone conversations, and also with their prompt responses. Joy's larger bed arrived today, just two days after I returned the medium sized one. I have again put it inside an old duvet cover incase it needs to be returned and have put her familiar smelling blanket and old cushion on it and after a couple of suspicious looks at me, a spot of sniffing and only a small amount of digging , it looks as if she's happy with her new bed! Success!
Thankyou all for your advice!