Newly assembled PC will not start

What GPU are you using?Could be PSU underpowered...

It's a very old (7 years) nvidia based GPU - I don't think lack of power is the issue - at least I hope so.

What model? You'd be surprised at how the power mounts up!
For example
http://prntscr.com/fvjsi

It's a 'LEADTEK GF PX6600GT TDH RT' with a geforce 6600GT. Just one video card.

The AMD CPU takes up 125W. I have not connected anything else (hard/DVD drive, etc).

Here is a related data point I found for this motherboard:

Power consumption will vary depending upon the hardware and upon the PC load.
The current consumption of the test PC with AMD Phenom II 710 X3 CPU, 4GB DDR3-1333 memory, hard disk drive and Cooler Master Silent Pro M 600W test PSU was without overclocking depending upon load:
- With Sapphire HD6870 video card the power consumption was between 111W and 227 Watt.
- With Sapphire HD4870 video card the power consumption was between 162W and 237 Watt.

Consumption with that GPU only 400W so not underpowered. I'm wondering if your BIOS is up-to-date for that CPU...I had similar results with a mobo that didn't support the AMD i fitted...

Listed under BIOS version F1...

Thanks for your responses dodger46. The bios code is executed by the CPU. Since the CPU isn't getting started at all, the issue of up-to-date bios should not apply.

I'm getting the motherboard & cpu replaced - hopefully the replacements will work.

Fair point. Guess the fact that the fans still run with the 8pin connector out would suggest a problem with the CPU or associated circuitry,and the only way to prove that would be to try another CPU. A very uncommon fault, I imagine, not met one like it, except when my young son bent some pins on the CPU, couldn't wait for me to get home... Presume your vendor will just replace both items and you'll never know exactly what was at fault, but if you do find out, post the result...