We've had our 2.0D auto for approx eight months, it has settled down to almost consitantly 42.5mpg. However, it is mostly used for out of town driving (normally non-motorway).
We use a VW Up for town driving.
Manufacturer MPGs are optimised for taxing purposes and are seldomly achieved in real life.
See here for real life data, suggests that you are not that far off the typical - http://www.honestjohn.co.uk/realmpg/bmw/x3-f25-2010