[MikkiJayne]

In normal filling the displaced air will just come up the filler neck past the stream of fuel. There are no issues with that. I'm fairly sure the problem happens when the fuel level goes past the level of the filler neck entry to the tank and because the vents aren't working, the displaced air then has nowhere to go. Parking on a slope allows the air to escape up the filler neck again hence why there is room for two more gallons.

Re the nipple thing, that operates a valve which is connected to both the top of the expansion unit and the charcoal can, and it's just occurred to me that it is probably operated by the fuel cap, given its location. I use it a lot in my A6 as when I press it with the filler nozzle I can get an extra gallon of fuel in the tank.

I wonder if it connects the expansion unit to the filler neck when the cap is off, but to the charcoal can when the cap is on? I can test that with compressed air tomorrow.