Its an engine management light probably if not flashing will be telling you that there is a water deficiency in the diesel or the filter is dirty.
Simply get some strong bleach and pour about a 2pints in the diesel filler(where you put the diesel fuel) , this will float to the filter on the diesel and clean it out and evaporate completely before it reaches the hot engine (make sure engine is hot first)
regards
Terry
Dagenham Motors
November 2008