Beko CDA751F freezer temperature only about -4 degrees C?

I have a 2 year old Beko CDA751F fridge/freezer which, at the max setting on the thermostat, reaches only about +5/6 degrees C in the fridge and -4 in the freezer. The compressor runs, cutting in and out as it always has. It just seems to lack cooling ability. I may have a problem with the defrosting system, as I have noticed a buildup of frost in the freezer lately - nothing too excessive - but can't understand why that would reduce cooling efficiency. I've manually de-frosted but cooling is no better afterwards. Does anyone have an idea what's wrong with the freezer? Andy