It may come down over time - but progress is pretty glacial currently.
Incentives are all pushing for increasing diesel penetration of the fleet (good for CO2 emissions) but appalling for urban air quality. At some stage the authorities will be forced to act to protect health and the regimes will change. If you want a car mainly for town driving, I wouldn't buy a diesel.
A lot of hope had been invested in successively tighter Euro standards reining in emissions resulting in lower pollution levels. In practice the test cycles were not good indicators of the ways that cars were used on the road, The recent remote sensing report on thousands of measurements taken in Ealing and the City of London last summer is fascinating.
http://uk-air.defra.gov.uk/reports/c...port_Final.pdf
Of course, Heathrow could help to improve matters by choosing to ban some or all of cars/diesel cars/ diesel buses/taxis from its premises and ensure workers/passengers rely on clean public transport. It would be interesting to see if this would give sufficient headroom for a third runway at Sipson.