Ok, Garage have carried out complete check and cannot find a fault.
They have pressure tested the system - no leaks.
They have tested the stat - working.
The feedback is the problem was caused by opening the coolant reservior whilst it was hot in reaction to the low coolant level light causing steam to rush out and air to be sucked in causing an airlock which then caused the overheating.
I've recommended to matt to replace the coolant level sensor as that has always been a bit sensitive.
The moral of the story - check your coolant before a long journey and then trust only the temp gauge to know if you have a problem or not. And never open the coolant bottle whilst your engine is at full operating temp.
Thanks for all your responses.
