Hi, have already tried a different PSU, so unless that one is defective too (not impossible) it's probably not the power supplies.
I couldn't short the switch pins on the mobo (combination of dodgy eyesight and poor access), but I did a similar thing and tested the switch operation at the mobo plug end with a mulimeter. The switch and associated wiring/plug are working perfectly. My PC doesn't have a separate reset switch.
I've put it all back together with original PSU, plugged everything in securely and I still have the same fault, so it's looking suspiciously motherboardy.
On the positive side, I have learned a little bit about PC troubleshooting I didn't know before....