Bosch Frost free freezer has frozen up - why?

My mother in law has a Bosch KDN30X00GB/01 Upright fridge freezer (freezer at top). The evaporator had frozen up completely and was encroaching on the fan blades making it very noisy. There was no air circulation as everything was blocked with ice. After manually defrosting with a hot air gun, I found the defrost heater & assumed that it had failed but testing it confirms that it is functional. It's rated at 240V/155W and I measured 377 Ohms resistance. The defrosted water drain was clear, the airways haven't been impeded and the appliance doors have never been left open. Without a circuit diagram, it is difficult to know exactly how the unit is controlled. There is a sensor measuring the temperature over the coil. I assume that this will just start & stop the compressor or is it involved somehow in the control of the defrost heater? I haven't measured the resistance of the sensor yet. Does anyone know what temperature element is used? If the problem isn't with the temperature sensor, I can only think that the controller is faulty. Does anyone know how I can check this & is there anything I should look out for?
Your assistance would be greatly appreciated.