The best thing that I ever learnt at uni in relation to tax law was the term "substance over form". A lot of people tend to get het up on specific rules (usually the no. of days one) and think that if they comply with them then they are away laughing. Unfortunately tax law (and pretty much any law for that matter) doesn't work in such a way. The IR will take a view of all of the available facts apply their own technical rulings and possibly then consider case law. This is the reason why you need specialist advice and you need to give your advisor the entire picture. It is even better if you can speak to someone BEFORE you embark on a course of action so that a plan can be put in place. They may even request a written ruling from the IR in advance.
David has given you a very thorough analysis of the types of things that the IR look for. Every case will be different and you can bet that the auditor may decide to apply different weightings to each of the tests in order to arrive at an overall opinion.