Ah, the small economical diesel myth! We had an 08 B Class 180cdi manual and that averaged about 40-42 mpg; fairly shameful given the claimed government figures.
I'm getting 45mpg on the motorway, 30 (heavy traffic)-45mpg (mild traffic) in town and get around 50-55mpg on a run to work early in the morning (20 miles of b road at a steady 40-60).