When I turn on the hot tap, the pump and burner seem to come on and there is warm water but then the pressure gauge shoots up past 3 and then the fail safe kicks in and the pressure falls and the error code 07 ("primary thermistor open circuit") flashes. The radiators do heat up but obviously not when the hot tap is turned on. Is it a case of replacing the hot water thermistor? Any suggestions would be much appreciated.