Good morning, my name is ***** ***** I will assist with your query today.
This sounds like a fridge thermostat fault, where the fridge is not able to accurately measure the temperature.
The fault presents itself as you described where there is a big variance in temperature to start with, due to the thermostat no longer being calibrated, and then results in a total failure, where it can either call for coolant constantly or as in your case tell the fridge its cool enough and not need to be cooled.
Please give me a few minutes to look up the cost of the replacement part
Unfortunately it appears the part is obsolete, but what you need is a fridge temp sensor not a thermostat.
I will continue looking, as if I can source the parts it would be worth repairing as the parts less than £30.
Sorry to say after checking with a number of parts firms none of them have a compatible sensor for your FF, I have also checked with Zanussi spares directly and they dont have the part for sale, so it is looking like the FF is end of life and you will need to buy a replacement.