Coolant temp sensor is on fitted as part of the radiator bottom hose close to the radiator hose flange you will see it from underneath it is a pain as to replace you will have to drain the coolant on this one. As soon as you pull the sensor you will loose coolant.
The sensor is held in by just a metal clip. pull this away from ht esensor and then pry up as it is O ring sealed it will seem to be stuck
problem turned out to be the coolant temp sensor as the car got to the stage where it wouldnt start cost £19 for the part and an easy job to do however that sorted i still get dodgy driving when the engine is cold