I'd be very wary of buying a new diesel right now, in view of the way the wind seems to be blowing. All the talk about diesels seems to be of increased congestion charges, increased parking charges, and exclusion from many city centres by 2025.
As Howard says, the latest petrol turbo engines are pretty fuel efficient; the 252bhp 330i touring automatic has figures that are quite eco-friendly; CO2 emissions of only 135g/km and an official combined cycle fuel consumption figure of 46 to 48 mpg (even tho' we know you'd get nowhere near that in real life).
