I have been told that my maytag fridge needs a new compressor at a cost of ?800 - the fridge is barely four years old. Surely this can't have gone at such a young age. Can you suggest anything.

If it is indeed the compressor and they want $800, and it is not a really fancy refrigerator, just get a new refrigerator. Did they check the thermal overload/start relay on the compressor? If you are really lucky, it could be that and in many cases is a DIY job. I'd hate to suspect a tech but I have seen this happen, they make a lot more on a compressor! Check your warranty for coverage of the compressor. It may be covered for parts but not labor.

Were the coils dirty under the unit? When they get blocked with dirt it will cause the compressor to fail. DrTom4444