Hi Wisp,
I also have an A6 2L Ultra (Quattro) Black Edition, brand new in October 2016. I too am seeing the same sort of MPG figures you are, I recently drove from Birmingham to Plymouth, then to north Brittany and back. I was purposely driving 'economically' but still with radio, climate control etc all on. I got an average of 45.3mpg for the entire journey. I was expecting well over 50 if i'm honest.
I did do a 2 hour motorway journey the other week in Eco mode (drive select thingy ma bob) with heated seats off, climate control off (except when the windscreen misted up - but don't get me started on that!), just the radio on... and I got 52mpg. Seems that the Eco mode really does work although..... the power is terrible even for the smallest of overtaking and when the rev's drop to idle when you lift off you think you have broken something lol.
Looking into it a bit more and speaking to my local main dealer, it seems with Drive Select in anythign above 'Eco' the figures you are seeing are normal. Granted it's a big car with alot of equipment but as you stated, I too had a 520D M Sport for a few months and saw well over 55mpg regularly... but... i suppose thats the price you pay for a car that looks so good....
I compare it to having a high maintenance supermodel as a girlfriend to a fat ugly slob of a woman that licks the smeg from between your toes whenever you feel the need! hehe