whats wrong with my electrolux er80341 fridge freezer?

The fridge gets too cold and forms ice inside on the rear wall at the top and on the left behind the salad crisper drawer. The thermostat is only set on 1 out of a possible scale of 1-6. The thermostat does not appear to switch the "whirring noise" off unless you turn it to 0 when everything goes off, is the thermostat not working correctly? Seals intact and working correctly, contents neither overloaded or near empty, door not opened excessively, appliance not located in excessively warm or cold room.
The freezer works fine, frost free, fan working fine, although the fan seems to be on all of the time.
Pulled the appliance out of the cupboard today, all seems fine, emptied, cleaned and left for an hour switched off, now back on again and fan has been going in freezer for about 6-7 hours, slight frost on freezer drawers, freezer empty at the moment, fridge re-stocked sparsely.
Help - why is my fridge getting frost in - my lettuce is frozen!