I wish I could say that's fixed it, but it hasn't, as I've already tried the old take the MB out trick, different power outlet trick, putting the MB in the freezer overnight trick (there was a cold solder joint on the MB - it cost me two week's pay at the time to have it fixed!).
All capacitors have been inspected for leaking and the BIOS chips have been re-seated.
I wanted to thank you for your advice though - it could well benefit someone else having trouble.
Mext step - test the power supplies - in the bath.
BOFH