The external sensor is connected to the Climate Control System. The external sensor went out on my XK. It was reading 136 F and it would get great AC but no heat without going to max high on the controls. I changed out the sensor and the system went back to responding to setting the temp/auto mode.

 

There is a DIY self test on the A/C control head as there is on my 96 X300 and may point to a fault . I believe the temp sensor is a thermistor and can be calibrated to read a specific resistance in a ice water and boiling water dunking

O C / 32 F = 5900 ohms

100 C / 212 F = 0190 ohms

The lower the temp the higher the resistance . Ruairidh , by removing the connector you mimic a lower temp which may trigger the heater to go high output . A shorted connector ( water ) mimics a high temp so the heater output goes low . Ruairidh , listen with the hood open the electric heater water pump to turn on with the key in the on position , engine not running . See page 45 and page 110 for the self test of the Doc. below :

http://www.jagrepair.com/images/Trai...des/703_SG.pdf

There is also a temp sensor inside the cabin that gets clogged and unresponsive