As said check the NTC (AKA CTS) this sits in between the plugs and provides the reading for the temp gauge and also controls the fans.

In any case this should only pose a problem when stationary/ in slow moving traffic for a while, under normal driving conditions if you are over heating then this is not your problem!

As you say the temp gauge always goes to half, I did hear that this is done via CAN bus as when the Foci was first released there was complaints of overheating, although as yours is actually overheating it should move further up the gauge!

Martin