Thanks for the ideas folks. Things are on a general hold now until tomorrow when the local PC shop opens. I'm going to give the USB card a go to see what happens.
stickyb
are you sure the usb is u/s? it couild be a short in the usb connectors in the case drasgging down the psu and therefore stopping the whole thing powering up. Check if he usb will work wihout using the cables to connect to the front of the case.
Done all that. PC fails to boot with the USB cable attached but boots ok when disconnected. However, all of the on-board USB ports seem to have died now?
frostbite
Also, check to make sure that no part of the case is touching the underside of the mb.
All spacers fitted but worth a double check.
mad_jock
Have you tried reblowing the BIOS? Or pulling its Battery for half an hour.
No, but i'll give it a go tonight.
It might also be worth a punt to try and boot from a live OS linux stick
Not familiar with Linux, nor indeed do I have the s/w.
I'll report back later with any results