Try throwing some more volts at the IMC/CPU Northbridge.
The problem is I've no longer got a board

I've spent so much time trying to get it working - two boards, three sets of RAM, loads of different settings, that I think I'll send it off to AMD, as that's what their tech support said to do. If they find a fault, I'm going to insist on the charges the retailer made be refunded, then I will either get another FM2 board or sell the APU.