Thanks again for the helpful replies.
I changed the Bios battery this morning and I was able to boot to the Bios.....for about two minutes then it died again.
Subsequent attempts to boot have failed. No beeps, just the fans running,
Considering it was very hot when it failed the other day I'm wondering if I have cooked the PCU, or at least ruined the heatsink interface which is why it booted for a few minutes after being switched off for two days. I suppose the only way to check is to remove the heatsink and have a look.