How can I fix my Mistral Frost Free Fridge Freezer?

Our freezer compartment on our Mistral ff71 heats up to zero degrees, then once we've pressed the fast freeze button, has to try far too hard to chill itself down again, repeatedly and even then it doesn't get back down to its proper chilled temperature. It is using a phenomenal amount of electricity and sounding like a tractor.
Has anyone found a way to fix this?