1 tenancy - 2 people

Tenancy Types and Management

I have 2 people on one tenancy agreement as they are a couple. They are on a standard 6 Months AST which is now running periodically.

My question is, should I put them on a new 6 month AST as a joint tenancy agreement? or should I continue with the current set up?

My main concern I guess is that I feel slightly exposed as a landlord without the 6 month tenancy agreement in place. They are good tenants and I don't doubt their willingness to pay every month but I would feel more secure if they were on a new AST. Also can you legally request they sign up to the new AST or are they perfectly in their right to refuse and stay on the periodic tenancy?

Your assistance would be appreciated.

thanks