It lives...
...in folds of red and steamy air
Anyway the problem was.... a burnt out power supply. My guess it was due to the bloody USB hard drive somehow surreptitiously frying the power supply. Annoyance and suckage.
But it means that the fix was easy, due to a nice little power supply provided by the curry-fed tethis. the motherboard was fine, as are the rest of the bits and pieces. So it was just plug in the new power and go. And bollox to the tech support people at work who refused to let me use their equipment to test the power supply.
The one drawback is the PC is a Dell and the power supply is not designed for it. It fits, barring the off switch (which may have saved the problem in the 1st place). So a pair of tin snips and bit of hard labour later and....
... it fits.