Also, a point to note on legalities of working for the same company for a long period of time is that it is the employers responsibility to determine your employment status.
Working for 1 employer for a nine month period exclusively does not automatically mean you should be employed, it depends on a whole host of factors, hence the employers responsibility.
The only person to gain from you being self employed is your employer as he will not have to give holiday pay / NI contribution / sick pay etc.
If you have a problem you should take it up with the employer first, and if your still not satisfied then an industrial tribunal would take place.