tanstaafl:
I don't think so. I only have a couple of USB things plugged in (mouse, keyboard and scanner). It's pretty barebones as is but I will remove all but the CPU/RAM/Video card to be sure. Even the video card and RAM could come out. At least a power up and some beeps would verify things.

tman:
I'm only shorting PS-ON to ground with the PSU connected to a motherboard. I'm well aware of the need for a load on switchmode supply.

The standby +5V is working since the motherboard standby LED is lit as it should be.

I do have another board I can try (although only 20 pin ATX and not 24 pin ATX). I'll try swapping supplies on both machines and see what happens. Hadn't done that yet because the wiring is all neatly bundled up and routed at the moment.

Is there a small microprocessor responsible for the PS-ON signal whilst in standby? I assume it must be since a board will generally still start without the main CPU won't it? If there is a small microcontroller no doubt it's embedded in some other chip making signals hard to get to. Not that replacing/fixing that is likely to be possible. Such logic could be fairly easily done without processing so I'm not sure about an actual processor being involved. I'll have to pull the board out of the case to look closer I think and maybe try and trace the power switch signal somewhere. Don't really like my chances.