Freelancer with only one client?

Cornwall2000

your main protection against IR35 (HMRC deciding you should pay tax and NI at employees rates) is your contract. I think you can get these provided for you for a fee that will also cover your legal expenses in case they are challenged. Have a google.
As others have said there are various factors the HMRC look at to determine genuine self-employment and you may want to consider these as well.