Any property which has a connected gas supply must be checked annually by a fully qualified contractor. Most common of all are contractors who are registered with GasSafe.
Every appliance in the property in the property must be tested including a pressure test to make sure there are no leaks in the pipework.
The tests are detailed and will cover how gas is connected, the correct operation of the appliance, how it has been fitted and whether there are any obstacles preventing the exhaust gas leaving the appliance safely.
When a tenant purchases an appliance and has it connected then it is their responsibility that the appliance has been installed within the regulations. Normally in the case of new appliances a competent contractor will connect & install the appliance. Any appliances which is connected & installed by the tenant must be checked first by a qualified contractor and a certificate issued.
A copy of the gas safety certificate must be left with the tenant.
Gas safety is of the utmost priority and tenants must be made aware of how important regular testing is and that there co-operation is required, to allow engineers access during reasonable working hours to conduct the test.
Unfortunately there will be tenants who ignore call for access, it is always a good idea to record when calls are made and formally put into writing requests for access. When a tenant is being obstinate and not allowing access, then the local authority can be called upon to see if they can reason with the tenant or failing that, a court injunction be sought to gain access to the property. This naturally is the last resort and a very rare occurrence.