SMEG CR330 Freezer at -35; Fridge room temp

Could you help with a SMEG CR330 SE/1 problem please. It is old, but a built-in one so a pain to have to replace.

Fridge on top; freezer below. Was working ok until two weeks ago.

Symptoms:
Compressor starts, runs quietly and warms up – output pipe is hot to touch.
Cooling fins on rear become warm.
Freezer goes to about -15C overnight and -35C over two nights.
Fridge only goes to about 2C below room temperature.
I can hear the refrigerant trickling back into the compressor.

Has been defrosted for 4 days. Door seals are fine.

From what I can see, the thermostat in fridge should switch off both the fridge and freezer, when the fridge is down to correct temperature. This thermostat had failed so I have replaced it and it does work, checked by holding a piece of ice against the capillary tube. Obviously the fridge isn’t getting down to temperature so that thermostat doesn’t trip, therefore the compressor keeps working and the freezer continues cooling.

I’m baffled how the fridge section is supposed to cool – cannot see any fan or air channel from freezer section; and no pipework visible inside fridge.

Questions:
How does the fridge section cool – Do the pipes go around the fridge behind the panels?

If it’s low on refrigerant could it be that it always falls to the bottom (freezer)section?

Is there maybe some other fault I haven’t thought of?

many thanks for your help,
G.F.B.