Sounds like you have done a lot of troubleshooting already..
but I would reseat the CPU, Reseat the GPU, I would pull all the wires off the motherboard such as speaker, HDD light, reset.. everything except power on.
Then see if you get anywhere..
thanks for the advice

didn't give any results unfortunately.
I've arranged with the shop to have it sent back & replaced, it's a free return anyway, so it's ok, was hoping to get a rig up before the weekend (& monday is a holiday here) but i guess i'm out of luck
