any ideas, not entirely sure where to look! Is it somwhere near the radiator fan casing?

Take out the black cover of the engine. Looking on the right side you will find the sender. It is mounted into one of the rubber water hoses with an electrical cable connected to its socket.

PS: do not do this on a warm engine, the boiling watter will get in your face and hands because of the pressure. Have a bottle of G12 handy, some liquid will escape.