Took the 24 (or could be 20, did not count) pin plug out, used paper clip on green wire and black wire next to it as per instructions and the power supply winds up big time.
Then took a piece of wire and put each end into the top of the plug (to simulate it being plugged in) and it fired up the power supply.
Then took two pieces of wire, put one end of each into the top of the plug and the other end on a microswitch... press the switch and the power supply fired up...
As you can imagine I was getting quite happy... last test needed.. plug the 24 pin plug back into the motherboard, then plug the two wires back in and tried the switch... power supply whirls up again, but no bootup it turns out.. also the issue was as soon as you stopped pressing the button down it stops.. I thought once it was connected properly it would fire it up and then all be ok getting a signal back from the motherboard, but when I took the wires out and pressed the proper power on button, the power supply starts up but completely differently.. no big whirring from the fan, and you can hear the hard drives start up, etc.. seems like the trick is in realilty purely to test if a power supply is completly dead..
My other thought was to see if I could connect straight to the physical dell switch, but not sure that will work and last time I tried to get to the back of the switch I accidentally pulled of the heatsink!!