You may or may not be aware that Diesel cars now have fitted an item called a DPF (Diesel Particulate Filter). What this does is help to clean up diesel emissions, and they do it very well as most modern diesel cars have almost zero emissions. In fact MOT stations had to upgrade their Emissions Testing equipment because of this.
The principle is that the filter, which is fitted to the exhaust system, traps the small soot particles and to stop it getting completely clogged up the ECU( Engine Brain!) does some clever stuff with the engine management to burn off the soot build up.

The problems occur when you are one of those thousands of drivers who only do short stop and start journeys, basically the cleaning cycle starts but never gets time to complete. From this the DPF will usually get blocked and will require replacement. Sometimes the warning light comes on warning that the process has not completed, but by this stage it usually has to go to a specialist or Franchise Dealer to run the regeneration manually, if this is not done then it will certainly block up.
Now if the DPF requires replacement you are looking at BIG money, but you do have an option now. There are a growing number of specialist companies who can remove the DPF (car will still run ok and pass MOT etc) and re-map the ECU to tell it to ignore the DPF, this is a much cheaper option, typically 1/3 of the cost.