Replaced thermister, but to no avail. Suspect it was actually the shutter at fault. But decided to buy a new fridge freezer. Shame because the dog used to love laying in front of the warm air outlet at the bottom.
Try defrosting it for 48 hours with the freezer compartment back carefully removed (so you can see the cooling fins). Although frost free, the defrost heaters break over time and you dont notice until it breaks. When removing the back, be careful because there is a thermistor attached to it.