Post up exactly the age miles 163/177bhp etc.
You said its mapped. What map. What power. Does it feel quick. When was it last serviced. Last check engine/turbo etc.
Lots of variables involved but its not right with the mpg. Before you ring BMW UK make sure all is 100%. They may not do anything without a full check up in the dealer first.
I average 44mpg and 550 - 600 miles per tank. Thrashed up and down the UK, 3000 miles a month. Did the same in my previous 163bhp 320d. However I am getting the car fully warmed up over 200 - 400 miles per day. Doing 20 mile journeys will never get the best out of the car.