serving Section 21

Possession - Section 8 and Section 21

I have recently given a joint tenant the chance to fill in a Letter of Surrender of Tenancy, which he has signed and returned to me in person following the break up of his relationship with the other joint tenant (his partner). They have been causing problems in the neighbourhood with their constant loud arguing and verbal abuse of each other in public (with some police involvement). This was my creative solution, to avoid going to court to get them to leave under paragraph 14 of Section 8 (I think), or inflicting them both on the neighbours until January 2013.

I now consider that the original 12 month AST only applies to the remaining tenant. I wish to serve a Section 21 so that I can make a new start with new tenants at the end of the current AST. The other tenant has, as far as I can tell, moved out.

However, I want to make sure that the Section 21(1)(b) I want to serve on the remaining tenant covers all possible contingencies. For example, it is possible that the tenant who signed the Letter of Surrender may change his mind before the end of the AST. So, my question is: should I put both tenant names on the Section 21(1)(b), just the name of the remaining tenant, or what should the wording be?

best wishes,
Tony