I can't quite believe it. I'm typing this message from my home PC. The one that was dead this morning. Well, taking everyone's advice, I took all the cards out, disconnected everything. After about an hour of messing around, I just happened to try removing the BIOS battery for a few minutes. Gosh, it worked. The fan came on, the POST test came on the screen. Amazing. I plugged all the drives back one by one and verified it was repeating the test OK. And here i am, back online.
However, it ain't all good. Pressing the 'power off' button on the front panel causes the pc to power down but the green light keeps flickering intermittently and is accompanied by an intermittent beeping. What could be causing this? A loose connection somewhere? Is it something i can fix? I know have to flick the off switch on the new psu to shut it all off completely. Anyway, tonight I'm happy just to see that basically the old dog is still working (for now anyway). Cheers all for kind assistance.