I dare say if you bypass that valve and just connect the pipes, that would prove if the valve is defective or not, it’s only air in the pipe (unless the car is upsides down).
Think I've now planned my weekend now. I don't get any warning light from the ecu so I'm going to blow through a few pipes to make sure there are no blockages. I'm beginning to understand how the EVAP system works from googling, but because fuel evaporates I can't find any reason why the whooshing sound when opening the filler cap, can't be the fuel tank venting into the filler hose. I assume there is no one way valve from the filler, into the tank.
