The essence of IR35 is quite simple.
If you supply your service through a limited company (known as a personal service company), is the manner in which you work and the type of work and relationship that you have with the client more like being in an employed position. If that’s the situation, then the relationship might be disguised-employment and you might be taxed as though you were an employee. Ouch!
Of course it isn’t that simple. There are very few client relationships that are cut and dried, and the problem is then acerbated because HMRC always seem to assume you’re guilty of trying to avoid the tax, and require you to prove that you’re innocent.
The government isn’t tempted to clearly define what is employment, because if they did, we’d all know what the rules are and then modify our actions to ensure that we always stood on the safe-side of the tax law. Instead, HMRC use their muscle and threaten/cajole, and if that fails, then it goes to tribunal and/or court.
There has been a huge amount of case-law since the seminal case in 1968, and you need to stay advised of the latest case-law and legislative direction to ensure that to the fullest extent possible, you sustain your client relationship in accordance with current best practice.