I've got a 56 plate 320D Auto (163 bhp) and the best I've seen on the motorway when taking it easy is about 48-49mpg. On my normal commute which is about 70% dual carriageway i get about 43-44mpg in summer but only manage about 39mpg in winter.
What I will say is that I think my car is getting better with miles, just coming up to 40k now and it's great.
I take it that you have had the MAF checked out? Maybe worth going out and thrashing it to death for a few miles, may work wonders