TD5 Water in diesel

Hi,
Just bought a 1999 td5 gs auto and whilst driving around yesterday we noticed that a light came on. We looked it up in the owners manual and found it out to be water in the diesel. Has any one come across this before and what is the best course of action? We had put £20.00 of diesel in early in the day bringing the fuel gauge up to just under half full and the the light seemed to come on after that. Could it be just a coincidence?

First of all Hi and welcome to the forum

Secondly i think you mean it has water in the diesel filter water trap, Change the filter for a new one and the light will go out

or Drain the old filter from the drain plug at the bottom and the light will go out.