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

Yeah, if you reseat everything, verify power cables are all connected, and nothing happens, send it back.
May want to try pulling the power cable and holding the power button down for 15 seconds. Probably won't help here though.