Cata larder fridge ice on back wall and not cooling properly

Hi guys i am after a bit of help please , my Cata larder fridge LF177Bl has started playing up .
Ice has stated to form on the back wall at the top and the fridge isnt as cold as before .
The compressor runs and the condessor coil is clear . I'm guessing it could be a fault with the defrost system but I cant see how to access these parts . There is no panels to remove inside the fridge and the back is covered in foam .
Any help please guys would be much appreciated

Could be short of gas sensors are probably foamed in at the back of the fridge

the foam at the back will not come off in 1 piece , if i break the foam will it affect the cooling

Start with a 24 hour defrost with the fridge unplugged and the door open - then see how it performs after that before you start ripping anything off.