The OP was regarding IR35, which was introduced in 1999 to stop the practice of IT contractors selling their services via their own private companies and taking dividends, which are taxed at a lower rate, instead of taking a salary and paying tax and NI. Often these contractors were employees of a Plc who left and were subsequently re-engaged as Self Employed contractors at the same company. This a large shortfall in Employers and Employees NI contributions being paid to the Inland Revenue.
Subsequent posts by the OP indicate that the question really relates to the difference between Self Employment and being Employees, which has been adequatley answered, although from the facts given above, it would seem the these people are Employees rather than Self Employed.
Do they complete the Self Employment section of a Self Assesment Tax Return?