It would appear that many UK motorists are driving about without Diesel Particulate Filters. The current MOT doesn't always pick up if it has been removed, and though illegal to drive a car without one it is not illegal for a garage to remove it. A change to the MOT will improve the situation, but there are also calls for spot checks to be carried out on diesel vehicles (as there currently are for checks on the fuel being used).
'Thousands' driving without crucial diesel filters - BBC News