The landlord has a legal requirement under the Gas Safety (Installation and Use) Regulations 1998 the Association must ensure that all of its gas heating and hot water appliances are serviced at least once in every 12 months.
The servicing of your gas equipment is extremely important for your health and safety and to others who share the building. The risk of carbon monoxide poisoning or gas explosion is a real threat, and annual servicing greatly reduces any such risks.
Only by allowing access to your property can you help to be sure that you are safe. The gas service will highlight any defects and the Association will ensure that they are rectified as soon as possible.
Failure by tenants to refuse or continually deny access is a breach of your Tenancy Agreement and entry will be forced to your home.