Hi all. Thanks for all the hints and tips.
To answer some questions there is no white smoke in the exhaust and not oiliness in the water that I can see
Plastic expansion tank under pressure as all have said.
I don’t know if the stat was checked and I need to find out as that’s really the next thing to get cleared off the list.
I’ll get the MOT out of the way this weekend first and then get it sorted out before the summer.