The temperature on the display used to be constant at -18 degrees C, but over the past couple of weeks it's started fluctuating between -18 and -12 over the course of several hours. The times when it is showing a warmer temperature are not necessarily when it has been opened; nor are they times when the room is warmer, either because of sunlight or daily variations in the temperature of the Aga (which also came with the house). There are times when it's showing -12 but the compressor isn't running, so it's not as if the refrigeration mechanism is struggling to achieve the temperature that the thermostat is set at.
Do freezers tend to show a higher temperature on the display when they are going through a periodic "frost free" thawing stage? How frequently do freezers typically do a frost-free cycle - daily, weekly?
I've not checked with a thermometer to see if the actual internal temperature (in a consistent location) varies - ie whether it's the display or the cooling which are varying.
I've never noticed our own Samsung freezer which we brought with us displaying a temperature which varies by more than +/- 1 deg C from the -18 that it is set at.