Judging by the times Diesel Particulate Filters are mentioned in your columns, your mailbox must clog up with them. My question involves the distance one needs to travel before the DPF sorts itself out. You say that short journeys are generally harmful, but how would you define a 'short journey'? I am thinking of buying either a BMW X3 or possibly a Mini Countryman but have mixed feelings regarding petrol or diesel. For journeys of say 10 miles or less, petrol is probably preferable but how much further need I travel to be fairly sure of the longer term health of a DPF?