Hotpoint Mistral Freezer/Cooling problem

Hi All. I first had a problem on the fridge, and thanks to this forum I managed to fix it, so now I have a freezer problem, this is the first place i came to!

The model is, I believe, a FF92 (all documentation has been lost in a house move!) and the problem is that the digital display is flashing the warning symbol and displaying a temperature anywhere between 11°c and 40°c (no error codes) – however the freezer is still freezing. I have put a thermometer in and it is current about -35° it is also becoming really iced up. It seems to me that it thinks it is warm so is constantly trying to cool it down.

From what I have read on this forum I think I need to replace the yellow and/or green thermistors. Is this correct or is it likely that there is another, more sinister, cause.

I look forward to hearing from you! Thanks!