ZANUSSI ELECTROLUX -- FRIDGE (TOP HALF) NOT WORKING?

9 times out of ten, when the freezer portion works, and the refrigerator portion doesn't, it is the evaporator fan that is the problem. I am not familiar with that model, so I do not know where the fan is located. But there is a fan that blows cold air from the freezer into the fridge portion. Usually, a fan blade is just stuck, and just needs to be repositioned or the bearing the motor spins on could be frozen or rusty. I little household oil usually does the trick, or you might need a new fan motor.

Another possible cause is the ice that can build up. If there is ice blocking the air's path it will have the same result. You said you had defrosted the unit, so this is not likely in your case

Short term solution switching the whole unt of for a day and it seems to work for a month or two till it happens again

If ice is buiding up its usually due to the defrost system not working. There is a timer located at the rear to the right of the compressor which every so often turns on the defrost cycle. To check it , remove the plastic box and with a screwdriver turn the clock around until it clicks. This is then on the defrost cycle. After a short time water should start to drip out onto the top of the compressor. If it does not , the heater may be faulty. If it fails to stop defrosting and move back to freezing , the timer is faulty.