The Inland Revenue used to use several "tests" to determine whether someone was self-employed and it has been discussed at length here:-
http://www.pprune.org/forums/showthread.php?t=193338
Customs and Exvise didn't seem to be so fussy but now they are one organisation, HM Revenue and Customs, they are trying to unify their rules so it may become an issue.
Quite often though, it depends on the individual Inspector and what you can sometimes get away with! Taxation legislation is not cut and dried and there are different interpretations to the rules - that's what has kept so many of my fellow ACAs in Porsches!
Cheers
Whirls