Chris at eSpares says...

This appliance has an auto defrosting fridge, If the machine is working correctly then the reservoir at the back of the machine will have water in it at least once a day. If the back panel inside the fridge is getting cold evenly the problem may be the thermostat capillary has become detached and it’s not recognising its cool enough, This would not be repairable. If there is only cold at the top part of the 'cold wall' then there may be low gas or an inefficient compressor. These faults would require a 'Systems engineer' to repair.