i'm 90% sure that's the water in the fuel filter light, time to change the fuel filter, but for now release the drain on the filter and drain out the water,
the drain will be a black hand knob, with a 3 core wire on it, be carefull as the wire goes very stiff due to diesel being dribbled down it over the years, and the cables can snap easily, if they do snap, your water in fuel sensor will not work, but if the wrong wire goes, then the light will stay on all the time.
Gazz, April 2010