Section 21 and date Notice should expire

Tenancy Types and Management

I issued an AST for a room in shared house on 6th Nov 2006. Subsequently the tenant moved to a different room. I did not issue a new tenancy agreement. I m assuming this is now a periodic unwritten contract on an AST. Are the terms and conditions of the original agreement imported into the unwritten agreement? What date should i serve a Section 21 to expiry. He is supposed to pay his rent on the 6th of each month, he doesn't it varies from about 9th to anything upto 24th. What date should a Section 21 expiry on a 5th?