Hotpoint ff92s freezer problems (please help)

Can anyone help me? I seem to have a different problem to all the other topics on the hotpoint ff’s and have yet to find the correct answer.

The fridge is fine, nice and cool, the freezer is very very cold but the lcd is reading anything from -7 to +22.

The freezer had started to ice up a little so following advice on the other forums we defrosted for 48hours, took off the back plate hoovered under everywhere ( found out the hardway a few years ago what pet hair can do!), checked doors, light etc

Nothing untoward was found(!).

Have turned it back on all lovely and cool again, fan working when door shut etc, but the lcd is still reading bizarre temps for the freezer.
Another thing I have noticed today is when I close the fridge door the lcd will suddenly change to a cooler temp for the freezer i.e.-7. but then change back within a few mins. +8

Should I buy a new thermistor for fridge/freezer?
Or anything else I should be loking at checking.

I really appreciate any helpful responses.

Or should I just turn a blind eye till it stops being cold and starts getting warm :o) ha ha.