F codes are none resettable on these boilers, so trying to reset the boiler will not clear it. The thermistor is inside the case, that is part of the combustion cover, so under no circumstances should a none gas registered engineer EVER take off a case as they cannot then carry out the relevant 26:9 checks for safety. The fault is indeed a thermistor fault, but this MUST be replaced by a registered gas engineer, as if a homeowner takes off the case they can cause serious problems, and possibly void household insurance.
Someonewhoknowswhattheyareonabout, January 2017