2001 Peugeot 306 cooling fan not cutting in

Engine Cooling problem
2001 Peugeot 306 4 cyl Two Wheel Drive Manual 52k miles

Hi, my 306 cooling fan isn't cutting in when sat in heavy traffic, the guage goes off the scale and the red stop dash light comes on. I have checked the fuse and it appears alright, please can you advise what I should look at next.

Thanks in anticipation of your response

Cornish Fossil