Turned out to be the thermostat! Replaced it today and all is working! (Took it out yesterday and did the test in boiling water and it didn’t work, so got a new one today and put it in so it’s all working again now!)
Thank you for the comment though!
You are welcome, suggest do the pressure tests as well just in case the overheating has overstrained the tank or a hose. Also note the comments on airlocks in the heater hoses, check when the thermostat is open, but as there is an air bleed pipe to the expansion tank, it should clear on its own eventually