I have have read all the existing posts at least twice. Not meaning to be flippant.

The light is really slow to come on - an eight count.
I have topped up the overflow tank.
I have replaced the overflow tank sensor.
I have remove and flushed out the overflow tank and yes the floating magnetic ball rattles.
I have replaced the temperature sensor.
The fan runs when the car is warmed up.
The fan runs when the A/C is turned on.
I am assuming the coolant module is in tune with the ECU??????

Any help or suggestion would be greatly appreciated. .