Why is my Hotpoint Mistral FF72 freezer compartment reading +43C?

Three days ago our Hotpoint Mistral FF72 fridge/freezer suddenly gave a freezer compartment reading +43C. We cleaned out a huge amount of dust from around the motor and transferred as much as we could to friends' freezers. Then the reading dropped down to the usual -22C for about 36 hours - now it has jumped up to +43C again. The actual temperature in the freezer is about -32C though.
Please does anyone know what is the likely cause of this? The fridge/freezer is just over 10 years old is it worth trying to fix it or should we get a new one anyway?

Thanks,
Tim
Pluckley
Kent