Bosch KGU 31125 GB freezer compartment at bottom freezes up?

Fridge/Freezer Bosch KGU 31125 GB
the problem appears to be a common occurrence with this model. The freezer compartment at the bottom over time builds up with ice and eventually pushes the bottom drawer out and the door will not close properly. I gather the ice build up is due to the defrost cycle not working. Ice will build up in the freezer, stopping cold air circulating to the fridge.
I have manually de frosted this twice and have ensured that the drain hole is always clear clear to the evaporator tray underneath.
Now I gather that the a) de froster heater/element could be open circuit b) the bi metalic sensor could be faulty or c) the de frost timer control module could be faulty. There are 2 de froster heater elements and I am getting readings with a meter on these but differing. The sensor I get a reading but not sure if that should be the case. I have a feeling it is the (expensive) de frost timer, I will have a look at that but thought I would ask the question first...

No error codes appear and the top part of the freezer compartment will freeze ice cubes fine and the refrigerator section will remain cool. I have checked the freezer fn and that works.