Advice with buying a Ford focus car?

I am interested in buying the new 2011 Ford Focus diesel. The problem is I do mainly urban driving with speeds of no more than 40 mph when not at work. I have a 30 minute commute to work 5 days a week with 7 miles of this being 30-40 mph driving and 3 miles of motorway driving. Occasionally I do more than 6 miles of motorway driving depending on work location. My main concern is the DPF regeneration. Is the DPF actively or passively regenerated on the Ford Focus 2011? I know there is a petrol version but the tax is �115 compared to about �20-�30 for a diesel. Would I be better of getting a petrol?