I fitted my replacement PSU last night. I think the mobo may be damaged. Windows was corrupted and so I reinstalled Win7 and had another go at overclocking. I managed 850 core on both cards at 1087mv which was furmark extreme burn mode stable at 1920x1080 4xaa, although I was only using 2 1200rpm GT's and the load got up to 65c, still within the comfort zone though and almost silent.

However, even after the reinstall I'm getting the screen go blank and then it restarts, this is randomly at the desktop with no overclock. It didn't like the latest nvidia drivers, as soon as I enabled SLI it went blank and reboot.

I've ordered a replacement.