A different Mistral problem?

I have a Hotpoint Mistral fridge-freezer, model FF73s, which has a slightly different problem from the one described previously.

The fridge temperature readout is +2, and the fridge (or its contents) feel about as cold as you'd expect with this reading, so it seems to be working properly. The freezer temperature readout though has become +46(!), alternating with an E2 error message. However, it still feels pretty cold in there; we emptied of all food except for a bag of frozen plums, and they're still rock-hard after 2 days of the error.

Should I go ahead and do a thermistor change as recommended in other replies? If so, should it be just the evaporator thermistor which E2 suggests is at fault, or should I change the freezer thermistor as well? And if I do the change, how do I explain it to my wife if the freezer stops working in the process?