I have a Bush TV, which I bought only about six years ago. It incorporates both video and DVD slots. When a DVD is inserted, the reader replies "incorrect disc", which it is not. I'm told that a replacement DVD reader is no longer available. Is this so?

More than likely as technology has moved on a lot since that set was made..

Have you tried cleaning the player with a disc ?
If your lucky a simple lens cleaner will do the trick/