Potterton Suprima 50, Temperature Sensor (thermistor)?

The Temperature Sensor (thermistor) has been suggested as a possible cause of my boiler going off just after a normal ignition sequence.
It looks normal, physically. Can anyone tell me if it is a positive temperature coefficient (PTC) thermistor, Posistor, i.e. it's resistance increases with increasing temperature. Or a negative temperature coefficient (NTC) thermistor, i.e. it's resistance decreases with increasing temperature. On my Fluke multimeter it measures 1.1Mohms at room temperature!
Thanks for any help.