Temp warning but no overheating

[Mossyboy1978]

Morning all

I keep getting regular temperature warning - the big red thermometer in the centre of the dashboard! - but the car is ALWAYS rock solid at 90 degrees. It's been happening for a about a fortnight and I've driven to good wood and back with the warning light flashing but the car was beautiful and never skipped a beat.

Could it be the sensor in the coolant tank?

The coolant level is fine.

Thanks

Mossy