Liebherr TP 1410 fridge not getting cold

I have a Liebherr TP 1410 fridge (about 4-5 years old) which is no longer getting cold. The light still comes on when the door is opened, but nothing else is happening. What is the most likely cause (thermostat? sensor?), and is it something that can be repaired/replaced yourself?

I called a couple of Liebherr “approved” repairmen, but it looks like the cost (labour and parts) would start at around £130 if it is an easy fix. An equivalent, new Liebherr fridge would be under £300 so not really sure what to do unless I can repair it myself.