Of course, the vaccuum is pushing down on the remaining body of fuel and not sucking it as I had thought because of the air intake whilst removing the filler cap.
No, you were right the first time Titus. As the fuel is consumed, it should be replaced by air. The intake of air you hear is an indication that this is not happening, and the vacuum created is trying to stop the fuel pump drawing fuel, although the pump is strong enough to overcome it. My parts list shows a two-way valve in the filler neck near the top connected to a breather tube. It sounds as if yours is blocked.
All cars that I have dealt with (at least later models) develop a vacuum in the tank as the fuel is consumed. Letting air into the tank creates a real risk of, at some point, reaching an explosive air-fuel vapor concentration. This can not be a good thing.
At any rate, the fuel pump should be able to easily overvome htevacuum and provide fuel to the carb(s).
What I find most annoying is that the nearly horizontal hose, from the filler cap to the tank, results in fuel sloshing out before the automatic cut-off of the pump can activate. On most other cars the steeper angle allows fuel to trigger the pump cut-off mechanism before it reaches the brim.