Thanks C12Froedman and unksol... I suspected that it was worth checking.
If CMOS can be reset by pulling the battery then yes, I did... all connections (plugs, RAM) have been reseated and the cards for graphics and network removed to strip the potentials down.
What happens is that I push the power button and the fan on CPU twitches and then stops - that's it...
So I guess I have to strip the CPU and reseat that - it's was on my list but I didn't want to go in there unless I had to!
Many thanks guys, I'll revert once I have some heat grease.