I have a problem with a Bosch Fridge/Freezer KGU3201GB07?

The problem with it is that the freezer will not cool to the temperature set, which is -24C, it only cools to -13C and the alarm is always on. I have put a freezer thermometer in and it shows the it is about -18C at the bottom.

I have spoken to the 'customer help line' but they are not technical, fine. But they won't put me in touch with somebody in their technical/service department even though they train their own engineers.
Has anybody a service manuaul or able supply a part number for the freezer sensor/thermostat.