Its interesting, but when you think about it, many registered gas fitters with qualifications have probably made mistakes as well as those without. What really matters is the quality of the work produced not if the person who did it is qualified or not. The presumptions is that if you are qualified you should be competent of turning out good work. So qualification is based upon presumption not proven ability. But companies get paid for their work not their qualification, its probably why a well qualified 3 month course person has problems getting employment with companies. The companies assume they have no experience of turning out good work.
The thing is you can't really know if they can turn out good work, until you see some of it. So what if you went to a site and saw good work from a none qualified person and good work from a qualified person. Which one would you pass if you did not know which engineer was qualified and which was not?
Obviously you would have to pass both.