Vauxhall Corsa water in fuel

I have had numerous issues with water in the fuel system. It all started several months ago when the engine management system showed an cylinder missfire. After diagnosti tests I have since had fuel pump, o2 sensor, fuel filter replaced. The ECU was even sent away to be tested and came back as normal. Everytime this fault occurs there has been considerable amounts of water found in the fuel system, At first I thought it was coming from the garage I filled up at but I have since changed fuel supplier. The car has been in the Vauxhall dealers and they have been trying to find out where this water can be coming from. The mechanic first thought it might be the fuel tank but this doesn`t seem to be holed or allowing water in. They seem to think that the Fuel filler neck could be the problem and water is somehow finding it`s way into the tank. They have also replaced the Carbon Canister which was found to be nearly full of water???? I haven`t a clue what this is or where it is located. I would welcome any thoughts or ideas as to where this water could be entering the fuel system.