There's a tool on the HMRC website that can help to define if an individual is self employed or not.
When Darren started working for me he was self employed, he had his own work, regular customers. He still sevices this work in his own time during the week. I do not supply him with all his work. He takes time off whenever he wants, at the drop of a hat, with no notice. He does not get paid sick nor holiday. He rents all tools required to be able to carry out his work at his expense. Any complaints, he rectifies this in his own time.
My accountant has said a signed invoice from Darren is adequate. It is simply a business expense. I have also spoken to HMRC about it. They have said so long as he invoices me then it's a business expense and his tax is his responsibility.