Image from the British Leyland Workshop Manual

Fuel is sucked from the bottom of the tank (1) by the pump (12) and pushed through the fuel filter (13) and into the carb float chambers.

The 'expansion tank' in the main drawing is not shown in the Parts List, and Clausager in 'Original MGB' only mentions the separation tank and the capacity limiting tank both as shown in the inset. The inset system could imply that although the capacity limiting tank would not be contain fuel when filling the tank, the 'airlock bleed' may allow fuel to enter slowly from the bottom until the level in the main tank and the capacity limiting tank were at the same level, thus reducing the overall level in the tank. This would be preferable to an external expansion tank which could result in fuel spillage into the boot if the pipe from the main tank were ruptured.