If he is not registered, report them to Gassafe and HSE. You can do it anonymously.
Try the local council, Building Control Department.
You must employ a Gas Safe Register approved installer to undertake this work on gas boilers / appliances. By using a ‘Gas Safe registered installer’ you do not have to make a Building Regulation application, the installer will self-certify the installation. In all other cases a Building Regulation application has to be made.
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