I dont know if this helps but my "water in diesel" light came on and stayed on last year. Luckily the car was still under its 5 year warranty so I phoned the RAC and they sent a van to my house. The guy actually pulled the pump out of the car on my driveway but only after he tested the diesel. There's a lot of dodgy fuel laundering in these parts so it's worth going to reputable, usually supermarket garages to fill up which I had been doing. He replaced it and the light was out. Next morning it was back on again. I booked it in with the dealers but between that and me going it went out again and touch wood it's stayed that way ever since. I cancelled the appointment.