WHAT CAUSES THE THE OVERHEAT LIGHT ON AN ASTRA TO COME ON?

I have a Vauxhall Astra estate R reg, 1600 cc engine

The engine overheat light illuminates when the engine/radiator cooling fan swithces on.

The water temperature goes down but the light remains on.

I get no other symptoms

I have just had a new thermostat installed to cure temperature problems caused by the old one which was half stuck in the closed position

does anyone have any ideas why this is and how to cure it