Hi Baz....welcome to the Club
I would agree with Greydog on everything he has mentioned and also sometimes it can be something as simple as the coolant pressure cap because if this is problematic then it can either not blow....hence the reservoir popping and not the cap releasing the pressure and also if it is weak, this can introduce a lower boiling point.
So for the case of either pressure testing it or even replacing it, it would be worth looking at before spending big money. Also the sniffer test for combustion gas in the coolant is definitely a must as also a cheap diagnostic test.
Temperature Infra-Red gun would determine where the hotspots are in the engine and show coolant circulation temperatures.
Let us know how you get on with it
Cheers, Trevor