My freezer won’t go more than 5 degrees below freezing. Thermostat seems OK so what’s up?

My Samsung fridge-freezer won't go more than 5 degrees below freezing. The thermostat seems fine and has the correct resistance for its current temperature.

What else could be wrong?

The display on the front shows the freezer at -20C (I'm in the UK) when the actual temp is only -6C which isn't enough for a freezer – it should be at -18C. The fridge compartment is fine and sits at the correct temperature of 5C above zero within 2 degrees or so.

There is no icing up. The defrost element works well (ouch) and there was no ice anywhere when I took off the cover in the freezer section.

What else should I check before calling out a (horrendously expensive) engineer?

Thanks!

EDIT: So I took the thermistor out of the 'housing' and exposed it to free air at the very top of the freezer and no difference at all. However, if I put the thermistor directly in a bowl of hot water, the freezer happily proceeds to move towards the correct temp of 18 degrees below freezing. What gives?! The thermistor is completely accurate for temps above 0C so could it be it only fails once frozen? Seems very unlikely! No idea what else could be wrong 🙁