Thanks everyone. I know we did the right thing, she started to lose the use of her back legs and it was breaking my heart to see her wobbling around.
The vet said he would have done the same had she been his cat, and it was very quick and painless.
Weirdly though, in the pub we were reminiscing and I said I wish I was still in touch with my friend, who gave her to me when she was a kitten. When I got in, I had a Facebook friend request and it was him. So weird.
Anyway, I feel relieved and although I am sad, glad that it's over.