By law, you must be on the Gas Safe Register to carry out gas work in United Kingdom, Isle of Man and Guernsey.
HETAS is the official body recognised by the government to approve solid fuel domestic heating appliances, fuels and services. Its work in approving products covers boilers, cookers, open fires and stoves and room heaters. It also lists in its Official Guide, factory made chimneys and carbon monoxide detectors and alarms suitable for use with solid fuel. You can search for registered installers on their website