How can I stop my Classixx KSR32623 constantly freezing?

My Bosch Classixx frost free larder fridge keeps freezing the contents of the fridge. Is it possible that the thermostat is now defunct? Can I change it myself? If yes, is it a simple job?
A few months ago the fridge was not evaporating the defrosted water.(I blamed the dog)(sorry dog).
For just over a week I have had frozen milk, fruit & veg.
The back of the fridge is covered with a fairly thick layer of ice so I assume that the thermostat is gone. Advice would be greatly appreciated.