SOLVED: I found the USB cable I was using wasn't working correctly.
Once I swapped it for another cable everything started working, but before hand there was a hideous amount of noise as if it was in a feedback loop as soon as I connected the UX2 up. (Noise above and beyond the noise mentioned before)
Output from the headphone port and the Analog outs is fine.
The other thing is the UX2 hates my Belkin USB hub, so by connecting it into my MacMini directly I've got no discernible digital buzzing and things are running as I expected them to.
I was about to throw this unit away, now I just need to throw my dodgy cable away.
Happy days.
Hopefully this helps other people in the future.
Tony