Since I replaced the head gaskets, my miggy has had no overheating problems with the temperature staying at about 95-97 deg when driving around or in traffic up to 100degC before the fans kicks in which brings the temperature down to about 97degC again, until tonight.

I took the back roads to my local DIY store to buy some decorating materials for the weekend. It is about a 10 mile round trip with all of the roads 30 or 40 mph speed limits. On the way there I noticed the temperature gauge was higher than usual at about 100degC and on the way back it was the same with it going into the red with the overheat light coming on when I was stopped at traffic lights. I put the heater on to full heat which started to bring the temperature down and when I started moving it returned to about 100degC. Although the evening was hotter than it has been for the last few days it is hardly summer!!!!
When I got home after turning the engine off I turned the ignition back on to monitor the temperature without the engine running and again it went into the red. I could hear an electric fan running to cool the engine and I checked the header tank and the level was at max.
When I rebuilt the engine I replaced the water pump with a QH one and also the thermostat as I had had overheating problems in the past.
The oil cooler has never failed and when I rebuilt the engine all of the waterways were clear, with no limescale where I live in a hard water area (I always boil the water for topping up to remove an much limescale as possible).
The engine was not running rough at idle or feeling excessively hot, could the problem be the temperature sensor?
Alternatively maybe one of the fans not cutting in?
The radiator was replaced with a new one about 40,000 miles ago due to a leak.
The car with all of its reliability issues is now beginning to live on borrowed time as I'm a great believer that there is only one thing to do with an unreliable car.... get rid of it.