Bosch Fridge Freezer – Fridge Icing Up

Hi Everyone, this machine was in the house when we bought it. The model no is KIM3002GB/01 FD7603 000179. I have no manual but I think it is a frost free model. I rang Bosch to get a User Manual as the model no wasn’t recognised on line. They told me it was at least 10 years old & didn’t offer me a manual.
The top third of the back wall of the fridge compartment has started to ice up accompanied by a slightly increasing fridge temperature.
I de-iced it but it came straight back within a few days.
I am thinking about replacing the thermistors as this has solved the same problem on a different fridge freezer in the past + they’re relatively cheap, normally accessible and can usually be done DIY without disturbing the coolant.
The door seals all look in good condition and the door closes fine.
In the fridge I can not see any panel to access through the back – it looks like one piece – no access. There aren’t any vent holes either to vent cold air into this compartment although at the bottom of the back wall is a collector and a drain hole for water.
In the freezer which is underneath the fridge it looks the same – no access – except there is the added complication of having the pipework for the coolant running down the back wall and horizontally under & above the boxes that contain the food so I’m pretty sure there is no easy access here.

Q) Is anyone familiar with this model? Do I have to remove it from the unit it is built into and access the thermistors from the rear?
Are the thermistors logical items to consider?

I am concerned that I’ll damage the unit (its not small) or even worse with the kitchen unit being chipboard that after 10 years it may have started to fall apart. In that situation I can see the wife wanting a new kitchen!! Advice please?