Expansion tank boils over

Good morning people I need some help please, I have a 51plate K series Freelander and the water in the expansion tank keeps boiling over but not all the time, the head gasket has been done I've changed the cap on the tank and changed the thermostat but yet it still does it could anyone please explain what's going on please and with a possible solution to solve the problem.

Thanks Guys

Hello and welcome try our Freelander section mate

Welcome to the forum.
Freelander section over ere: https://www.landyzone.co.uk/forum/land-rover-freelander.9/