If a new AST is not signed and in force when the old AST expires, in law it is replaced by a statutory periodic tenancy. This has exactly the same terms and conditions as the original AST but is ongoing - it has no specific expiry date. Should a tenant want to leave when such a tenancy has come into force, he is required to give one month's notice. If a landlord wants the tenant to leave, then at least two months notice expiring on a rent day (i.e the day in the month that the periodic tenancy commenced) must be given. Proceedings when rent is late - section 8 actions - are unaffected. A landlord may also increase the rent under section 13 if he wishes.
Many landlords (including me) never bother to renew AST's for this reason, and I will only issue a new AST if a tenant specifically requires a period in excess of two months of secure tenancy for some good reason. I have never been asked for this in 9 years as a landlord.
Letting agents however like to renew AST's as frequently as they can, ignoring this partiular piece of legislation. This is because they can make good money out of tenants and their landlord clients by doing so.
P.P.
"Ohm's law rules here"