An update on this:
The fridge was taking up to a day to get from 9 degrees to 5! All the time the compressor and fridge fan was runing, and the freezer varying between -32 and -35 degrees, which is far too cold.
I decided to try disconnecting the freezer fan to see what would happen. Now the fridge is fine, gets down to the required temperature very quickly. The freezer as expected has suffered slightly, with temperatures of around -23 at the bottom which is fine, and -13 at the top which is a bit too warm.
So it does seem that the fan is a contributory factor in the problem. I can live without the fan if need be but would rather have it working properly if possible. I don't want to call an engineer because it's not worth it, but am fairly handy with electronics and so am willing to try replacing the temperature sensors in the freezer if anyone thinks these might be at fault.
Please see the following picture of the freezer:
There are two temperature sensors, a black one and a white one as indicated by the arrows. The white one measures a resistance of 16 ohms when the freezer has been left to defrost for a while. The black one is completely closed circuit i.e 0 ohms, maybe some sort of temperature switch?
Would anyone be able to simply advise whether or not either of these items might be at fault in this case, and if so, where I might be able to obtain replacements?