Thanks people.
It's none of the silly things and all of the parts are about 6 months old including the mother board & CPU, the power unit and hard drives.
The power unit works fine in another computer and using a different pwr switch made no difference either.
I removed the battery for a few minutes. Bridged a couple of pins (I forget what they're called for now) as per instructions and the computer turns on but goes off immediately. So at least something happened. That's as far as I got about 3 weeks ago. Would cost me about $40 per hour for a diagnosis. I can replace a lot of parts for less than I'd spend in a shop and may even do that.
Somebody tell Shroomz the name of that Scottish site please. They'll be more than happy to translate for him. Jock???