There is an airlock in the coolant system.
Get the car up to temperature while standing so that the cooling radiator fan kicks in twice. Remove the filler cap slowly as before and leave it off. Top up the coolant to the correct level.
Leaving the car ticking over, proceed to the next step.
Look at the left hand headlight side of the engine bay as you are standing in front of the car.
There is a plastic flat head screw at the top left side of the radiator. This is the bleed screw...
Undo it slowly until you see coolant leaking continously from the radiator without any hesitation, it should be a nice steady flow.
It should take quite a few full turns of the bleed screw to see the coolant start to appear but dont remove the plastic screw completely, just be patient. Once you have a nice steady flow, retighten up the plastic bleed screw up and top up the lost coolant again. Let it tick over until the radiator fan kicks into action yet again and replace the filler cap. Job done.
Just be mindful of the temperature gauge. It should reach just below 90 while moving after at most 5-6 miles (8-10Km) of normal driving and go slightly above 90 when sitting in traffic... If it doesnt do any of these and sits at a lower temperature than that, your thermostat is siezed and needs replacing as soon as possible. That will be the initial reason why it overheated and let air in the system