As a Landlord you have duties under the Gas Safety (Installation and Use) Regulations 1998 to arrange maintenance by a Gas Safe Registered engineer for all pipe work, appliances and flues, which you own and have provided for your tenants use. You must also arrange for an annual gas safety check to be carried out every 12 months by a Gas Safe Registered engineer. You must keep a record of the safety check for 2 years and issue a copy to each existing tenant within 28 days of the check being completed and issue a copy to any new tenants before they move in. See the Health and Safety Executive web site for full details.
Reasonable rates charged for both Estate agents and Landlords.