To install a boiler legally, you MUST be corgi registered,
To install a boiler legally, you MUST also be CITB registered
To install a boiler legally, in most cases you must also be registered for electrical work. This can be with 6 or 7 different organisations, not just NIC.
Ask for cards or verify with the organisations. No excuses; if you have lost your card and are waiting for the new one to arrive (this does happen) corgi, and any of the others, will be happy to confirm that the installer is legit when you call head-office. Any genuine installer will be or at least should be delighted when you want to make sure he is not a cowboy.
A good indicator of the quality of the installer is his answer pertaining to –supplementary bonding- a hot item on electrical inspections. Very simple job to do if it is not adequate (almost never is) and any techno babble invariable indicates shifty attitude. All it takes is connecting all the metal pipes in the bathroom with at least 4mm wire, and connect it to the cpcs.
The installer must commission and notify the boiler which means you must get the benchmark filled out, complete with name and corgi number. You also will get a confirmation from the council that the boiler is installed legally. These are not extras, they must be done.