I'll give it a go - someomne else has also suggested that I take the DVD out and give it a clean (don't know why I didn't think of that one) and also clean the lens on the DVD reader. I will report that once I have tried everything...
Tried all of it... Still doesn't work. Read up on a US Lexus site and it seems that a lot of 02 IS300 owners had the same problem with the Sat Nav. There doesn't seem to be a simple solution - probably DVD laser or laser driver gone. Will take in to Lexus and see what they say but not holding out for much...