I wouldn't, for any price, argue with buildersmate, but at least here in the U.S., any fridge/freezer, be it side by side or over/under, only has one compressor and this directly feeds the freezer section. You should notice a little door device (called a diffuser here) in the fridge section that would normally blow cold air when the machine is running. This little door is modulated by a thermostatically controlled device of some kind (depending on the make) which varies how much it's open. This is all that controls the temp in the fridge section. A blower motor in the freezer section not only circulates the cold air in that side but forces it through the aforesaid door to the fridge section. On occasion, i've seen combos with two blowers to more efficiently distribute the air, but still only one compressor. Obviously, in deference to the venerable (and well respected) buildersmate, things could be different in the U.K.
The problem is usually a malfunctioning diffuser or its thermostat.
If you don't have an ice build-up on the back side (inside) of the freezer section, the auto-defrost is probably working fine. If you do... I'll explain how to remedy this in another thread..
Best of luck!