I assume you getting the fridge temp from the display, assuming the display is correct (the fridge is actually 15 degrees), then its unlikely to be a sensor fault, since the fridge knows its not cold enough and should be calling for coolant.
In the first instance I would look to defrost both fridge and freezer encase the coolant channel has frozen and stopping coolant from the freezer compartment.
Another potential fault is the fan has failed, and therefore not circulating the coolant.
If the fan is working and after defrosting the fridge your still having an issue, its likely to be a fault with the control board.
Please let me know how you get on defrosting and checking the fridge fan.
It is marketed as a frost free unit in terms as the specs, to defrost to clear a blocked coolant vent which is what I think is causing your issue you need to power it down with both freezer and fridge doors open and then apply warm / hot clothes to them inside to raise the temp to 20 degrees. I would expect this to take a number of hours, but is the cheapest option as an engineer would cost £50 to come to site.
Thank you for letting me know Guy, I await to hear if this has resolved your issue.
The coil should be a defrost element, how is the fridge doing today (a few days on), have you removed the wedge from the fridge vent?
Ive checked and it is defiantly conventional compression coolant rather than evaporation method which typically doesnt use a heat sink.
I dont have a service manual for this model, so reliant on memory, it could well be the actuator for the vent has failed, are you able to remove the vent and see if you can see the solenoid, and check if there is a voltage across the terminals?
There are three sensors on the fridge, humidity and ambient temperature, and coolant temperature, which allow the fridge to determine when to start the defrost cycle, start the fan or call for coolant. I would trace the sensor wire on the back panel and find out where they connects to the control board, and then see if the sensor is short or open circuit using a multi meter on continuity, is the sensor you have located under the fan?
H Guy the control board is usually fitted behind the control panel so in your case in the door panel.
It sounds like the sensor half way down the door is a thermostat, however without tracing it back to the control board its hard to be sure, but this sounds the most likely purpose.
With regard to removing the housing this is something unique to every model, normally there are little logs so you squeeze the casing and it will slide to one side then pull out, as stated earlier I would suggest you get an engineer to site as these plastic components do not come off easily and the tabs are easily broken.