Out of interest when you are sat in traffic with your heater on is the AC also on?

When the AC is on the fan should be running so that could be the reason why.

I think it could be the coolant temperature sensor though.
 
No I never have the aircon on, to be honest, don't really have the climate control unit on at all unless I start misting up.

I'm convinced it's all linked to the wiring on the sensor reading 1.18v and the other side of the sensor reading 5v. :(

Looks like I've got a busy weekend... Just don't really know where to start diagnosing it.