Double Barrel, you mention about the fuel selector switch. When this is set to 'Both' the supply pipes from each wing tank are joined together, before they go to the Carburettor. So if there is any difference in height of the tanks, the fuel will drain from the higher one into the lower one.
I parked at Shobdon one day with one wheel in a hollow, when we returned from our lunch, the left tank was overflowing and the right tank was down to half full. The fuel only moves slowly between each tank, so if that happened in flight, you would be holding progressively more aileron for S+L flight.