These are some pics I took of the pipework. They actually doesn't help at all with understanding how the vent system works, or indeed what the problem might be, but maybe someone else will be inspired...
The vent lines from the top of the tank. The small one is the front right, medium (blue stripe) is the front left and front upper combined, and the large (blue and yellow stripes) is the upper rear.
This is what Elsawin calls the 'expansion unit', shown roughly in the orientation it is in the car. The fitting to the top right of the pic is the top surface and is horizontal. This contains the gravity valve, and feeds to the valve for the vent release in the filler neck. The upper of the hoses to the left is the one which goes to the charcoal can.
In this view you can see that the upper rear tank vent goes straight to the top of the filler neck.
Top three go to the tank, to blue/yellow, black, and blue in top-to-bottom order.
Hose for the charcoal can
This is the pipework from the charcoal can. Top right is the line going forward to the N80 valve. Lower middle is from the filler neck assembly.
Closer view. Right is the pipe to the filler neck. Left is the vent to air from the can. This is a closed system with no openings other than these three pipes.
Given that the main tank vent goes straight to the top of the filler neck I don't see how that can be the cause unless the hose has collapsed. Any filling solution (petrol pump or gallon can) is putting fuel in below where this pipe attaches
If the charcoal can is blocked then that could block the lower tank from venting via the expansion unit, but the top of the expension unit can vent in to the filler neck through the vent valve so its still a mystery.
I think I need to burn some fuel off and then pull the driver's side arch liner out and check all the hoses are intact. If they are then I'll disconnect the charcoal can in there and see what happens.
Alternative suggestions welcome though...