I can recount instances such as descibed by Penhouse. Not only gas inspections, but the same for periodic electrical installation tests,. A good one is for communal water testing for legionella when the dwelling is independant not part of a communal system.
Michael Read is more than likely right in the most part with his speculation. It is perverse that lo-no quality adminstrators can threaten court action, let alone achieve eviction when most standard tenancy agreements alow the landlord the right to 'break in' where safety is the issue. Then charge the tenant of associated repairs.
It could be that the 'contractor' directed mail to the address(s) for the attention of 'The Occupier'. I usually trash and shred such items without opening.