I have a Zanussi ZX99 which is about 2 years old. The fridge got warm so I followed the advice to check the freezer heat exchanger which was iced up. After defrosting the fridge works again for about a week until the heat exchanger starts to ice up under the two ports that take cold air to the fridge. (but the centre area is clear of ice).
Working upon what you have described I would suggest you have a problem with the defrost heater. Either this is duff or you have a problem with the defrost timer which is not supplying the power to the heater.