Scandinova Fridge Freezer – fridge problem

Hi
This old but serviceable FF7054D Fridge freezer has developed a fault for the first time in about 20 years! It’s used as a back up for freezing veg and beer and cider storage in an outbuilding …. Sadly it’s started to freeze my beer! The fridge compressor (it has separate comps for fridge and freezer) is running constantly even if I switch the fridge control off , which turns off with a click and switches the fridge light off. There is continuity between terminals 3 and 4 even when off , the compressor doesn’t run if I disconnect the brown wire (3) . Is this likely to be a thermostat issue or is there anything else to check, before I try to find one.
cheers
dan