Hi, I just fixed my Bosch fridge freezer which had the same problem - ice building up at the bottom of the freezer. I found a forum which suggested that I should empty the freezer and defrost for 24 hours - so that the blockage would clear. I did this BUT at the same time I had the unit switched off and dragged it out from the wall. I cleaned the metal grill at the back with a hoover crevice tool to remove all of the dust. I removed lots of dust from the wall, floor and under the unit. Then I found the problem. There is a black condenser unit (round metal) at the base of the unit. On top of this is a white plastic tray into which water can drip. (I guess that the condenser generate heat which will evaporate the water). There is a plastic pipe which runs from the bottom of the freezer to this white tray. This pipe was blocked with dust, lime scale, and horrible gunk. I carefully twisted and removed the pipe. Used a long screwdriver the clear it out. Ran it under the bath tap. Note that there is a sleeve over the pipe to protect it from the heat of the fan unit - since the pipe runs under the fan. Then I got a small electrical screwdriver and cleared some more black gunk from the outlet at the bottom of the freezer. As I got further in I reached ice. So I left it to thaw and drained the water from the pipe occasionally. I also cleared dust and lime scale from the white plastic tray as this as where the dust got into the pipe. It's done the trick and the freezer is fine now. I'll also mention that my electricity readings went sky high while there was a problem so get it done soon. Hope this fixes your problem. If you need more info post here and I'll try to help.