It is a legal requirement that any work carried out on domestic-sized gas appliances used in a commercial setting must only be carried out by a Registered Gas Installer.
A Registered Gas Installer is an installer or service engineer who has completed the required gas installation safety training and who has the necessary insurance to carry out gas installations to the legal standard.
If you have a domestic sized gas appliance, Insist that your builder / developer / installer provides you with a copy of the 'Declaration of Conformance' for your gas installation. A copy can be purchased on the NSAI website which confirms that your installation complies with the necessary standard. You should keep this document in a safe place.
Industrial/ commercial sized appliances/ equipment should be installed, serviced and repaired by a competent service agent for that equipment.