It's only between the plugs if it's an SE, if it's a blacktop (By that you are saying it's a 1.8 or 2.0?) it's on the right side of the head, they are both NTC sensors and measure the temperature of the metal directly thus if you do decide to change there is no need to drain the coolant, in theory if it is thatI should think your fans will be on (As it uses the same info) as well as the all important input to the ECU for fuelling requirements.

An easy test is to disconnect the sensor and turn the ignition on, this should run the fans as it indicates a fault has been logged, once reconnected the fans should not run.

Are there any signs that it is actually overheating?

PS this thread really needs moving to either Technical Q&A or the Focus room.

Martin