There's a small trap door at the top of the filler neck that opens when you insert the fuel nozzle during filling, but is there another trap door further down the filler neck that prevents fuel from coming back up? Like a check valve?
Reason I ask is I tried filling the tank last night and I could only get a couple of litres into it before the pump would shut off. I tried a different pump, same thing. If I tried trickling fuel in it would spill over. Fuel tank is empty.