how can I fix my Bosch exxcel fridge freezer KDV20VOOGB?

There seems to be a problem with the defrost sensor not working properly, the fridge section at the bottom warms up and the small freezer section at the top gets colder.
The freezer is iced up behind the side panels and needs defrosting manually every couple of weeks. Is there a part that we can replace so that it defrosts automatically? If so, where can we obtain the part from? We have scrolled through the parts suppliers but have been unable to find anything resembling the sensor part.