on fiesta's there is a know problem with the fan! you need to check the relay and the fuse! the normal givaway for this problem is that the reverse light doesn't work either!

The relay in question is the GREEN one!

I have used water wetter and found that helps (makes the cooling system more efficient).

Also check the thermostat is working properly, and it is the correct one.

My toy is a bit over cooled now! so I don't normally have to worry about that anymore!