My husband and I ran our own Engineering Consultancy and we used sub contractor Engineers and Designers. All of these people were limited companies and needed to be able to prove they were not working for any particular client for 2 years or more. We were always mindful as a business that we did not fall into that trap either. We were advised by our accountants, and it was always something that was openly discussed as it is common practice to work as a contractor within the oil and gas industry.
To mitigate any instances as a business, we would always issue defined contracts to our contractors outlining the contract term, thus no open-ended agreements, and they submitted timesheets and invoices on a monthly basis. We could not impose working practices on them, other than appropriate dress code for the office - so hours and days worked were entirely up to them. If they wanted to be off one day, they would just advise us. Their holidays were never recorded because we were separate companies and no answerable to each other.
We were advised it was necessary to maintain defined independence between organisation and subcontractor.
Hope that helps. But as I stated to Nikki it is always best to check with your accountant because case law can change the meaning of IR35.