Hi Colin
I would first check the coolant pressure cap as if this goes faulty and allows the system to lose pressure then the boiling point of the coolant will be lowered which leads to overheating.
Also, it can in certain circumstances allow air to enter the system, again leading to overheating and also poor circulation of the coolant
Also, its the cheapest and easiest part to replace, so I would definitely start there.
Let us know what you find
Cheers . Trevor