Denon ADV 500sd

I was having trouble playing DVD's but only near the end of a Movie, at the start everything was fine but towards the end of the movie it would freeze up. So I took the cover off and found the problem.
On the plate that goes over the DVD is a plastic insert which rotates and in the centre of this is a metal washer with expanding clips on it to put pressure on the disc. Well this part had failed, not the plastic bit but the metal disc that sits in the middle. Can you help me with the name of the spare part I require and where I can buy one from.
Thanks
malcolm-harnden@ntlworld.com