If the temp sensor has gone, the fan isnt gonna cut into cool the coolent is it?

coolant temp sensor is nothing to do with the fan switch.

You wanna check the coolent temp sensor, not the thermostat

its the fan switch that makes the fan cut in. i suppose if the stat is stuck shut then it wont allow much water to pass by into the rad, but the car would be getting hot all the time not just when stationary/ in traffic etc.

if its running cool when on the move then its probably a fan switch. my gsi never used to cut in till 100 degrees then it went past 100 for a few weeks till it cut in (gradually got worse)
and one day the fan switch failed altogether.

remember vaux corsa fan switches dont cut in till 98 degrees anyway.

could be that, though i thought thermostats generally stick in the 'open' position therefore it will just be freezing all the time.. correct me if im wrong.

my fan is only cutting in when my temp gauge gets to 100
it has always come on just before 100
am guessing thermostat is half stuck shut so only a bit of warm water is getting into radiator?