To switch on the ATX PSU one shorts out the two PWR/S pins on the mobo, this should then send pin 14 of the PSU 20 pin connector on the motherboard low (active state), which switches on the PSU. (This much I have been able to find out from various pages on the web).
Has anybody got a clue as to what is likely to be in the mobo circuit between these two points? I appreciate that it may differ from manufacturer to manufacturer. All I know is that my mobo is an ASUS, model unknown. and I can see a point near the front panel connector pins going from 3.3V to 0V when I press the power button on the case, but pin 14 is not dropping low.

The Power switch on the CP block near the front of the board is the connection (supposedly direct link) to the ATX connector. That is where the Power Switch makes that connection and thus makes the short. Now, as you have noticed that there is a distance from the Pwr Swt and the ATX connector. Should a piece of wire or LAN that is inside the MB open or burn out it will stop that swt from completing the connection. This will make it impossible to repair much less start the PC.

Thankyou, AJ?
My problem is that I believe that it is not just a simple track from the CP header to the ATX connector. If it was, I could simply put a thin wire link between the two. I can still do that, but I'm worried that I would be circumnavigating some on-board protection circuit that I know nothing about, and cause some real damage. But thankyou again for your prompt reply. Regards John