I would probably start by bleeding it to release any air trapped, i normally do this by starting the car up from cold with the header tank cap off leave heater on 1 let it warm up once up to temp squeeze the water hose a few time should see bubbles appearing in the header tank wait till fan kicks in and cools it down.

if this dont work i would probably change the thermostat, u might as well change the fan switch as its not much.

is it the car in our sig coz it looks a beauty

hope you get it sorted