I have peugeot 306 d turbo and a warning light has come on it says ive got water in the fuel filter?

Ive got a d turbo only had it around a month and a warning light came on i looked in the book and it says its water in the fuel filter its the light on the top left, as im no mechanic i dont know if this is a serious fault or something stupid,can anyone help do i need to take it to a garage or is it an easy diy job? how much will it cost roughly? to be honest any help at all would be much apreciated thanks
Answer
Hi the most likely problem is that the sensor on the diesel filter housing is faulty. it is a round sensor on the back of the housing which unscrews. second hand sensor are around £15. also i would advise that you change the filter too