Normally at the start of a tenancy it's a 6 month contract and called an Assured Shorthold Tenancy. That is now almost universally the most common type of tenancy agreement. It's valid for 6 months for additional security, thereafter it continues on a monthly basis.
As Dedworth says, as long as you've got a written statement from the landlord that the agreement will continue and that you have permission to stay there then that is sufficient to meet the rules.
