Pug 306 Overheating - Any advice please ?

Hi Folks,
My 306 Meridian overheated today in traffic and the ubiquitous red "STOP"
light came on.
The temperature gauge was indeed showing much higher than usual.
First thing, I checked the oil which looks nice and clear and at the right
level, so hopefully the engine has not completely blown up just yet.
At speed it seems quite normal, so I imagine that the radiator thermostat is
working OK, but perhaps the fan(s) are not coming on when they should. I
don't remember hearing the fan.
Interestingly, I tried it in traffic with the air con switched ON - No
problem.
It has always seemed that the cooling fans are controlled differently when
the air con is off. Is there a separate fan thermostat (nothing to do with
air con) I should be checking ?

Any advice gratefully received....

Cheers,

- Steve