About damn time, luckily i've not been a contractor for some years now and have missed the IR35 shitshow of recent years. Having a client offering a contract having to determine if my company is an actual business or i'm just a disguised employee is a ridiculous way of doing it. Sorry but when i was contracting i was also taking on other freelance work, had long standing retainers for maintance for past clients and having junior developers working in my company doing the grunt work, thats a business, i'm not a disguised employee.
The responsiblity should be on the person to argue with HMRC that they are a business and for those legitimately operating as such, then its not at all difficult.
Regarding maintenance retainers: I have been in a loop where customers realize they haven't "used" their monthly retainer and dump requests on me in the last week of each month. Did you experience this? Would love to come up with a tactful solution to this.
The responsiblity should be on the person to argue with HMRC that they are a business and for those legitimately operating as such, then its not at all difficult.