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Hello There and thanks very much for your answers. The vet said pretty much the same as you suggested jollygreen, that Harvey must have either had a fright or maybe tried to stand and got a pain. As it happens I was cleaning his litter tray not so far away and I shook a carrier bag open (you know the way you do with an empty carrier bag just before putting something in it) , even though it wasnt something that had ever made him jump before he had his accident maybe its left him feeling nervous at sudden noises and thats what scared him. It was just so out of character for him as he's usually so laid back. He's been acting quite strange since it happened and keeps going right to the far side of the underneath of the bed (by the wall) so it obviously upset him whatever it was. Its so sad seeing him unable to run about and play, he was so active and playful. Apparently its an injury that takes a fair while to heal. We're giving him lots of extra love and attention and even though the vet says he will always have a limp hopefully he'll be back to his chirpy self soon.