I had a very interesting chat with a service manager at BMW Battersea whilst collecting my car. I have a 12-month-old new model X3 3.0-litre diesel. I do a low mileage for a diesel (about 9000 miles a year) and about 30 per cent of this (in mileage) on short journeys. I asked his advice about protection of the Diesel Particulate Filter. He firstly said that it should be incumbent on sales staff to warn customers when buying cars of the possible problems associated with low mileage diesels, and this very often isn't done. He said that it is most important, as you advise, to give the car a good run at least twice a month and he said it is much more effective to put the cruise control on and run the car at a constant 50mph (as opposed to 'blasting it'). He also said that not only does the filter get damaged but also, with the build up of carbon, there is a fire risk danger in some models of car. He naturally said that BMWs were amongst the best for longevity of the particulate filter but the above precautions should be adhered to wherever possible. I thought this was very constructive.