Since my post I did a bit of net searching and confirmed what you say about the glass having to be heat reflective.
I also found the web site of a glass manufacturer who supplies the appliance manufacturers, as it were - www.schott.com
but I doubt these people would be interested in an enquiry from joe public.
I can't imagine I am the only person to have had this problem. The cooker was bought just before Christmas 1998 so is just over 5 1/2 years old. It seems hard to believe that others have not had to replace oven glass in the past.
I've contacted numerous glass companies out of the Yellow pages, and of the 2 that do fire-resistant glass, neither could say for sure whether it was suitable for ovens; one said it was suitable for stoves, but I'm guessing that means it is far from reflective; in fact probably the complete opposite !