IR35 also has this interesting point...
https://www.simplybusiness.co.uk/kno...self-employed/
substitution – could you bring someone else in to complete the contract, or do you need to do the work yourself? If you can’t send someone else, you’re likely to be within IR35
In other words if you have to turn up to do the work yourself they may consider you are employed. If you can send someone else you might be self employed. Problem if you need someone's unique skill set.