I am a gp carer and my gs's dad went self employed, remaining in the same job. He cannot supply alternative workers, has no equipment and has to be ferried in the company van. He works where he is told to so is not genuinely self employed with his own business. It was an avoidance tactic but yours is different as he actually does have employees.
He got a nil contribution decision when he had been earning £60,000 pa, now insisting he earned £5 ph in the same job.
Given the long history of avoidance tactics - CMS took out a liability order, sent bailiffs, it was with arrears and enforcement etc - I requested Fraud look into it. This was a struggle I will not lie.
I know it was a difficult one for my worker. But I asked him to try and achieve a lump sum settlement. This was because £5,000 was due in arrears even after bailiff collection etc. I said I would settle for a lower figure and Fraud said they were surprised when he emailed agreeing - after 10 days of daily phone calls. The team worker told him he was not happy with his "books" and he needed to submit more paperwork.
It was in my account the same day. Perhaps I took the easy way out but it was better than the stress with CMS and continual changes in the decision process.
You could try this route but it will be dependent upon past compliance. You can go over your case workers head. I found out who was head of CMS and emailed them. This resulted in my matter being passed to Financial Investigation or fraud.
I sense your frustration op. Good luck.