Do I serve a section 8 or a section 21?

Tenancy Types and Management

I have tenants who are over 2 months rent in arrears. Some of that will be paid in part through Housing Benefits. I don't know when that will be and haven't been offered any part payment by the tenants. I'm aware that my contract is with them and not housing benefits but the tenants feel I should be ok about waiting for payment. I can't afford to do this. How is it best to recover the arrears and also gain back posssesion of my property? It was home to myself and my children before I rented out to the same tenants in Oct 05.