Liebherr Fridge freezer CUN 3033 problem

Hi I have the above fridge freezer (5 years old) for the past few weeks the superfrost alarm is flashing almost constantly, the right hand (as you look at the panel) green number on the panel is flashing on 3 the left hand number is on 5. The fridge freezer seems to be at a fairly constant temperature although occasionally stuff in the freezer feels a little soft and at one stage there was a more obvious ice build up in the fridge although this seems to have settled now. The water container on top of the compressor has been filling up to the top and not evaporating off, I have to empty it. I have had an engineer out who replaced the sensor/control panel? at the front of the fridge this has not solved the problem. I telephone Liebherr who suggested that it will be a faulty evaporator sensor (part number 9590206) The beeping is driving us potty and the fridge freezer has been banished to the downstairs toilet until it starts to behave itself! Does it sound like the evaporator sensor and is this relatively easy to fit yourself with some basic engineering knowledge? Any help greatly appreciated.