fan switches are notoriously crap and because they are set to operate at such a high temprature
you can replace them with either a mondeo diesel or a sierra deisel version,, both of which cut in a little lower
the one to look out for is the red one, but the green one is definitly fro the sierra 1.8 TD



the tell tale signs of an expiring head gasket is a gurgling orm the heater matrix. this can also mean that the pipes ar the wrong way round on the water pump though, so don't be too alarmed