With current generation diesel engines you save pennies and potentially spend pounds. But not, generally, until the diesel is between three and six years old. So fleet owners and users benefit, while the private buyer who gets the car afterwards is the one who pays the price. All Fiesta 1.6 diesels use engine management to achieve the same as a DMF, and Ford is working on this with other engines. What can be achieved depends on the engine.

I have talked with their powertrain man, Andrew Frazer, about this. No petrol turbo needs a DPF, just a catalytic converter. Dan Harrison, my website editor, is running a Focus with the 1.6 Ecoboost. www.honestjohn.co.uk/our-cars/ford-focus-estate/wh.../