I have just been messing around with the Fireface UC trying to find out why the above problem (see post #13) is occuring. I found that if I turn on Link (Gang) mode, as described on page 46 of the printed manual, the problem goes away. If I turn off Link (Gang) mode, then the problem comes back.
So to clarify: if I want to adjust, say, line output 1 level (mono) and then change the headphone level (stereo), Link mode must be first turned off to adjust line output 1, and then turned on to adjust the headphone output. If Link mode is left turned off before adjusting the headphone level, then the level and pan and incorrectly adjusted.
Perhaps it would be a good idea for the Main output (usually headphones) to ignore the Link mode so that when Link mode is turned off to adjust mono channels, it does not have to be turned on again to adjust the Main output (which is always stereo anyway). There may be a good reason why this should not be implemented, but right now, I can't think why not.
Jonathan.