landlord signatures on tenancy agreements etc

Tax Planning and Financial

Hi,
On an AST (or other tenancy) where there is more than one landlord (ie the property is owned by more than one person), is it mandatory for both owners/landlords to sign the agreement, or can one sign on behalf of them all?
Where a landlord istructs an agency to let and/or manage a property, are there any requirements for when the agency signs the tenancy agreement on behalf of the landlord?
What happens in the case where the agency is instructed to sign on behalf of a property owned by more than one landlord?
For a common law tenancy agreement with a fixed term, (eg where the rent is more than £25k a year), at the end of the term do the tenants have to move out, or does it work like an AST where after the fixed term, the tenancy becomes periodic until one party gives notice?
Thanks for the advice
Best
Bruce Alsen