You may have seen this site:
http://www.refrigeration-engineer.com/forums/showthread.php?4129-Help-Needed-Fridge-not-working-on-Hotpoint-Mistral-FF91
Nearly the entire section is devoted to exactly your problem and most identify as you have the problem is with the Thermistors.
Be sure to compare your model to the ones in the discussion since there appear to be 3 of the devices intalled... 2 in the freezer and 1 in the fridge. There's are U.K. links (I'm in the U.S.) for ordering the Thermistors.
Additionally, there are online sources for Owner's Manuals for yours and usually a YouTube instructional video if you want to search that site...