It is a legal requirement under Gas Safety Regulations 1998, that the landlord ensures any gas appliances, gas fittings and flues provided for tenants are safe, you must also have a Gas Safety Certificate undertaken by a Gas Safe registered engineer at least once every 12 months. You must issue a copy of the Gas Safety Certificate to the existing tenants once completed, or to any new tenants before they move in. You should keep a record of each Gas Safety Certificate for at least 2 years as may be asked to produce it in the event of an issue arising.