A couple of months ago my fridgemaster fridge/freezer thermostat broke and I had a new one installed. Now a couple of months later the fridge is icing up at the back. The thermostat is on two and can't be lowered. I am defrosting it every day but no sooner I have done so than it freezes up again. It is far too cold.

Look into the self defrosting system for your problem. Timer,heater,defrost bimetal thermostat. Any or all of these could be bad.

Is the freezer compartment iced up?
Or just the rear upper section?
If the freezer is iced up the defrost heater my be faulty.
When the new thermo was installed maybe a wire was left off for the
Defrost heater?
If the top rear near the coil is iced you may have a low refrigerant charge.

Most often the timer that tell it to defrost is the 1st culprit to check out,, when you find it, you can use a screwdriver and manually turn it and put it into defrost,, turn it till you hear a hard click,and stop, after a few minutes you will be able to hear the heating element sizzling ,you will have to open the freezer door to hear this. That is the defrosting in action,, the little motor tends to go bad ,that turns the timer itself,,

Hi on the fridgemaster fridge/freezer is there only the one control for both fridge/ freezer temp and is it the one within the fridge part