Friend wasn't too keen on a complete reinstall - so I have had another bash at removing nVidia dregs (even after Driver Cleaner and Driver Sweeper there were still files and registry entries, and a failing service hanging around), I also removed all the ATI software, updated mobo, sound, network and storage drivers and then reinstalled a different ATI driver (a couple of versions back).
We still got another STOP 0x3b afterwards but we noticed it only seemed to relate to when the computer was not in use and conicided with entering sleep mode so I have disable Sleep power settings - fingers crossed.