Apparently it was an intermittent fault with the PSU. It was giving out enough juice to power the rig but it only worked when it wanted to.
They wired the PSU up to another PC (a cheap and nasty Media Centre job) with on-board video and sound. Sure enough it POSTed 4 times then gave up the ghost on the 5th attempt and wouldn't work again after that.
They installed a new PSU (Akasa Powerplus 650W Quiet V3 Black) and they tell me everything is now hunky dory
I can confirm the AMD rig is now fully functional again Just a couple of hiccups on first boot this afternoon, but a visit to the BIOS sorted them.
Just like to say thanks to everyone who made suggestions and gave ideas, and thanks for putting up with my rants
As John Cleese once said:
"Technology frightens me to death. It's designed by engineers to impress other engineers, and they always come with instruction booklets that are written by engineers for other engineers - which is why almost no technology ever works."