Cheers ShyTorque, I think that's it.
As ST pointed out, it could be the hard drive, but that wouldn't stop the system progressing through the POST check. I've also reseated the DIMM's, Video Card and swapped the Video Card around for another, so I am quite sure that leaves the CPU now.
The clincher is this however; Out of frustration I tapped my knuckle against the top of the heatsink and it booted up! It did promptly fail again, but still, a fair indicator I think.
Cheers,
ASI