I've just been to Barmouth and back with the family today (wife and two children), over very hilly single carriageway roads.
130 miles and 44.8mpg (by the OBC)!
Not bad for a 335d. It was mainly quite sedentary driving though we overtook quite a number of Sunday drivers who were admiring the views.
My usual average is 34.8mpg (2008 335d MS with efficient dynamics)