Miele K8867-S1 freezing Biofresh compartments

Miele K8867-S1 2006 model Nr 60/73490478 Type: 16208 4
The first sign of a fault was the fridge not cooling sufficiently, then after finding behind the rear panel was iced up, a 3-4 day defrost was started. The drain was clear and no dust was on the external pipes behind the fridge. Following the defrost, the fridge worked normally for a few days and then began to freeze the contents in the biofresh compartments down to below zero (between -1 and -3 degrees). We have tried turning up the fridge and biofresh compartments to maximum setting (9 for both), the fridge temperature increased over the next 24 hours, but the biofresh compartments remained below zero (should be zero). After 24 hours the temperature returned to normal (4) in fridge but the biofresh compartments were back down to -3 degrees again. The fan is working (top of rear panel) and the compressor sounds normal and runs for a short time and then stops. We have checked with our energy supplier and our usage is very similar to last year at this time, so we don’t think it has been running much more than usual.
All sensors have been checked with a multimeter and the readings seem alright. Please can you tell us if there is an actual thermostat in this model, if so where is it located?
We would like to know if this is likely to be a sensor or thermostat type problem that can be fixed at reasonable cost or is going to be something much costlier that would cost more to repair than replace.