Having looked at Bruces video, and several others I have got myself a Compaq PDP-123P PC power supply. This does not have an on/off switch, since the wall socket it plugs into does have a switch this did not seem like it could be a problem.
I've separated the wires by colour, bundled all black (GND) wires together, and all yellow (+12V) together, and hooked the green (PS-ON) to a black (GND). When I turn on the power at the wall the fan does not start so I am lead to believe there is nobody home at this point!
There is a grey wire, a post on another forum suggested this is "power ok", and the poster said it needs to be connected to ground OR 5V red wire (he wasn't sure which!), so I'm wondering about this.
Also some videos show a white cube resistor (20W, some apparently use several of these) being connected to 5V red and Black ground wires to provide a load on the 5V circuit.
No smoke has been released at this point so it can't be too major a problem!