Just one of these thoughts when your having a good sleep then suddenly in the middle of the night something flags up and thats your sleep for the night. The garage told me they fitted a new sensor and it was in the bottom hose. Have they fitted the wrong sensor? Surely if the sensor was in the bottom hose it would not give a true reading until the thermostat opened. Sensor must be fitted to the head somewhere me thinks. Unless you know otherwise. I will keep you posted.