Tenant can't pay rent

Possession - Section 8 and Section 21

Two tenants held joint AST from 01.07.13 for fixed term of 12 months. One of tenants has informed me that he is no longer able to afford rent, has put this in writing, moved out and returned keys. The other tenant has told me he cannot afford rent either, in part or full. He initially said he would move out but now says he has nowhere to go and has been advised by Shelter that he can stay in the property. The rent is paid monthly and is due again on 2nd March - do I have to wait until the rent has not been paid for two months before I can serve notice? And do I serve section 8 or 21? All advice gratefully received. Thanks