Earliest Date for Section 8 Issue/ Would the Courts Accept a Section 21 Unsigned by Tenant?

Possession - Section 8 and Section 21

I may wish to serve Section 8/ 21 notices on my LHA tenants who have a periodic tenancy created from the mutually agreed extension of their probationary 6 month AST some two years ago.

The tenants have paid their rent in arears, for every four week period ending Sunday, on the Friday following.

Eg their last payment was made on Friday 02/09/11 for the four week period 01/08/11 to 28/08/11.

However they have missed their payment due on Friday 30/09/11 for the four week period 29/08/11 to 25/09/11 and have remained unresponsive to several attempts to contact them by text and phone.

What is the earliest date I could issue a S8 notice?

The S21 notice that I served upon them after the tenancy became periodic was not signed by them. Would the Courts be likely to reject the S21 if the tenants claimed falsely not to have been served it?

Thank you for your assistance.

Tom D