Nardi Fridge Freezer NFR36X

This unit is ten years old , a stand-alone fridge over freezer.
About five years ago it was serviced and re-charged
It is working well but it has recently developed an annoying habit – inside the back of the fridge ice builds up. (Evenly, over whole width )
In order to maintain the correct temperature 5deg. I have had to set the thermostat fairly cold >3
If I set it slightly lower, it does not ice up, but the temp sits at >6-7 deg.
I checked the magnetic door seal. It seems to be okay. (If I slide a paper around the perimeter there does not seem to be any gaps, but neither is it =very tight)
I checked the level and it sits square on the floor.
The freezer section is okay (it does not have separate controls, but the temperature is subzero)

I deal with the problem by manually defrosting once a week – by turning thermostat down for an hour until the ice slides off the back. I wondered if the machine is supposed to have any other control which cycles it to defrost?
Any ideas welcome.
Rollo