Technical problem with radiator fan

[The Viking]

I have a radiator fan in my x reg 1.6 which will not come on when the engine is getting too hot. The fan works OK when 12v are put through it and teh fuse is ok, so I'm guessing it is some problem with a temp sensor or the thermostat.

Anyone any ideas or can tell me where the thermostat or a temp sensor are located on the engine?

Thanks in anticipation

The Viking