Hi there

I am not a domestic engineer but did a domestic course about 4 years ago ,from what I can remember your fridge freezer operates of one stat.
Your freezer relys on your fridge section operating the compressor.A problem a lot of people have is when they put there f/freezer in the garage over winter the fridge never rises above 5C so the freezer does not operate and rises to the same temp as fridge.

So I would guess your T-Stat is out of calibration and operating around 12C instead of 5C.
N.R.S do an equivilant stat I thinks Its a VL9 but check with them