yeah as above it's a Cylinder Head Temperature Sensor rather than coolant temp sensor....
the guage is driven over the CAN bus / SCP link and only is programmed to stay in the middle once up to temperature unless the temp varies dangerously. This was apparently because there was a large number of complaints of 'high running temperatures' in foci, despite the lack of a scale on the guage!
Ok so that probably doesn't help, but a failed CHT would probably result in rough running when cold. Have you checked fuses and relays? Also worth testing the fan itself directly off the battery....
Chris