Before attempting thermostat fix, check to see if, when you open the door to either the frige side or the freezer side, the circulating fan is running. This will be, of course, when the compressor is running. Listen for it.
Here in the U.S. (all fridges work about the same) the cold air generated by the compressor process enters only the freezer side, but is then moved by way of a circulation fan to the fridge side. The "door way" thorugh which the cold air moves is controlled by a modulating motor connected to a lever that opens and closes the door responding to demand. This little motor fails and the door won't open, hence the freezer side will be cold, the circulating fan will run, but cold air won't reach the fridge side.
Alternately, the circulating fan may be inoperative, so if the compressor is running and you don't see or hear the fan running when you open the door, that may be the culprit...