Well well, thank you! Sure enough it was that, and I never even use surround (I am a musician who uses my external firewire interface for recording and it’s always been set to stereo) however recently the driver for it kept crashing and it appears that during some auto-reinstall it had set it to 5.1! I have just set it back to stereo and confirm it works.
Sorry for entering it as a bug when the fault lies at my end, but thanks for the help and ‘fix’. 