Well after a fair bit of testing drivers under Vista I've finally have the system running how I'd like it - although not with Vista.
I've installed windows XP x64 instead of Vista and using driver 191.07 the system is behaving perfectly. All 4 GPU's working and no errors other than VLARKills so far after 2 days.
So annoying that they can't or won't fix the Vista/Win7 bug in the drivers......
