Reply to "Some good...some bad! It passed the MoT test ... But it over heated on the way home."

You want a full cooling system. You want it full when cold. You want it air tight. As the engine warms up, the coolant heats and expands in volume. That heated, and expanded in volume, coolant needs to go somewhere. The expansion tank is so named as it is the reservoir for the heat-expanded volume of coolant that is created once the engine is up to operating temperature.

It collects the coolant and holds it until engine shut off. As the engine cools, the coolant volume in the system shrinks, creates a vacuum and the coolant collected in the expansion tank is drawn back into the cooling system.

Then the pattern repeats itself.

Larry