Thanks for your reply, Doc. I've had a look and yes, there are 2 sensors on the thermostat housing, one brown and one blue. My 406 doesn't have aircon.
With the weather being so hot, I got the 406 up to temperature idling in the sun and discovered that both fans do come on after all (very quietly), at a pretty fast speed, but only when the temperature gauge is nearly at 100. The fans go off when the ignition is turned off, whereas I am sure that on our Xantia they used to run independently of the ignition circuit. 100 degrees seems a bit late to cut in. I've found 3 relays in a triangular black box in between the fans. Perhaps one has packed up and that's why the fans don't run on their own. But why 3? Are the fans supposed to run at 2 speeds?
The old BXs were much easier - just a thermoswitch in the radiator that you could easily by-pass and have a cabin switch in an emergency.
Anyway, thanks again
Ps What is a Bitron? Sounds a bit like Citroen.