Thanks to everyone who responded. In an attempt to sort out the BIOS, I removed the CMOS battery for a few minutes and replaced it. After doing that the PC wouldn't even power up at all so I admitted defeat and took it to a professional. It turns out that it was a failing power supply. It was failing when I had my BIOS problems and it was probably coincidental that it finally died when I fiddled with the battery. All is well now. Thanks again for the advice.
Rgds