The problem with not reporting them, can have and no doubt will have a detrimental effect, i.e. will or can be more dangerous. Although I can completely understand peoples reluctance to report an installers work, "he was a very nice chap" "he was cheap" "he's a friend" "I don't want to drop him in it" and all other manner of reasons.
What you have to remember is, before working on your property he was working elsewhere, it may have been one of your friends or relations house and he come to you by recommendation. So they may now also be at risk. He's also moving on to his next job now and possibly putting that customer at risk. Having been advised by so many in this thread, that
you should report this matter as a matter of urgency.
You also have a duty of care to these people where he may now be working. By reporting the installation to Gas Safe, they can insist that he provides a list of all addresses where he has worked. Gas Safe can, if they feel it is appropriate, write to everyone in a certain area and advise them of the installer's poor workmanship, telling them that they may be at risk.
Gas Safe may, depending on what they think, suggest that the installer needs just a little bit more training in his chosen field. But when
you go to bed at night you need to sleep peacefully.
You do not want to be reading about the installer in the local newspaper and how he has been charged with manslaughter and then say to yourself "I could have prevented that 5 year old from dieing, if only I had called Gas Safe"!!!
By having a Gas Safe inspection