Hi Keeley, Im having exactly the same problem as you with exactly the same freezer. Its driving me potty.
I have looked into it on various sites and the same answer keeps popping up.
On top of your freezer above the lights and super frost button is a plastic cover. Under that cover is a circuit board, it is a Thermostat. Ive found out that you this part can be purchased from a company called ukwhitegoods.co.uk
The part number is 546039402. £51.69
Make sure you un-plug the freezer before changing
Chris, November 2007