I have an Indesit fridge freezer, however the fridge part keeps freezing up on the back wall and the food inside partially freezes. I defrost it but it happens again, I have turned the temperature right down to 1 but it is still the same. Could this be a thermostat fault, if so, I know I can get the part but would it be easy enough to fit, or would I need to call out a fridge engineer?