mistral ff92s temp problems

Can anyone be of assistance. My fridge Hotpoint Mistral FF92s is showing room temerature and freezer is -22 as normal. I have read most of the posts related to this problem and think I have a thermistor problem.
When the problem first appeared the cooling fan would not run, therefore I defrosted and when restarted the freezer went back to -22 quckly with the cooling fan running. The fridge also began to cool, although very slowly but eventually, the fan stopped(probably after a few hours) and the temp would not go below 14. I have eliminated the door switch/lamp, motorised flap-which is open and I have seen moving and I assume the fan is ok.
When you turn off at mains and open freezer for a few mins, the fan will run again, hence I think it is to do with thermistors.
Can someone confirm which thermistors I should get, and how many. I cannot see any in the actual fridge cavity and 2 in the freezer. Also is it normal for the fridge to take far longer than the freezer to get to temp?
I am a service engineer on catering appliances although not refrigeration and therefore do not want to have to call someone out.