On the way home today the gauge was close to 130deg C, so I popped the hood. The upper coolant hose is not super warm, I can easily hold my hand on it - and there are no signs of boiling when I remove the lid on the radiator. Not even a lot of steam. If I rev the engine i can see the water being pushed into the radiator, so I suppose the flow is good form the pump.
..Must be a sensor that does not align with the gauge.
..Next step put a thermometer into the radiator when the gauge is at 90deg or above.
Get a laser thermometer, not expensive (I got one from Amazon for about £10 recently) and gives you an accurate reading of various points in the engine bay. Also you don't risk scalding yourself taking off the radiator cap!