As others have said, the car will need a good long blast on the motorway to get the DPF to regenerate which may help a bit, plus it delays the costly replacement.
Ive got a 2007 320D. Mainly used for getting me too/from work and shopping, all only a couple of miles and I probably get a similar average to you. But once or twice a week I do a 100 mile trip on the motorways where it gets 50+mpg. They just hate stop start town driving
