Thanks for your suggestion.

I've tried that - same result. The psu fires up, the screen comes on to a mid grey screen and then nothing.

It's failing its hardware checks at startup and therefore not even getting to the point that it's looking for a system. What's curious is that using the on board video rasterises the screen, yet using a known working nubus video card doesn't. It all points to a problem on the logic board for me.

Cheers!