Good reason for NOT using local garage

On the way home from the shops today (in torrential rain) the "water in diesel filter" warning light came on.As I was close to home took it in to the local garage.Mechanic checked and opened the drain valve and said there is no water it must be a faulty sensor.Drove to the garage where the van is normally serviced and M.O.T'd to ask  them to order sensor.The manager said shall we check it first just to be sure.Mechanic checked and drained off at least a quarter of a coke bottle full of water and diesel !!Drove home with no warning light.