Radiator thermostat switch
 

It's quite common for the radiator switch to fail, the tourmax replacement is over £40, and most of the cheap eBay alternatives switch the fan on at too low a temperature, so before you replace what appears to be a failed switch take the old one out and thoroughly clean the threads and mating face on the switch and radiator with scotchbrite or similar,to ensure a good electrical contact when fitted, it worked on mine today.

Good info, thanks.