After trying a few more suggested fixes by PCS (reseating power cables), somewhat to my surprise, their last chance suggestion of clearing the CMOS (as suggested above, by shorting the clear CMOS jumper pins) seems to have cracked it. I have no idea why this worked, or indeed how the fault happened, I did nothing between shutting down on Sunday and trying to boot up on Monday, and I have made no changes to the BIOS.
Anyway, having cleared the BIOS settings back to factory defaults, I am not sure if there any changes I should make to optimise this model of PC?
Anyway, having cleared the BIOS settings back to factory defaults, I am not sure if there any changes I should make to optimise this model of PC?