If you look at tech bulletin 014 it states that to check on work completed by a non registered person you would be expected to expose pipework if it is buried in concrete screed,, lift floorboards, climb through roofspaces etc,so its a lot more than a cursory check and more than what most customers would call reasonably practicable, especially if you''re whipping up their newly tiled floor.
It also mentions (to my chagrin as fuzzy will be chewing my bum) "No person shalllcarryout anywork in relation to a gas fitting unless they are competent to do so" and "the HSE defines competence as a combination of both training and experience.Therefore anyone who does work ona gas fitting must be competent to do so, whether or not they are required to register with gas safe register"
It seems to me the HSE doesnt know who or what it wants doing as far as defining who may work with gas fittings based on its own definitions. One day it may actually define once and for all who is allowed to do what in what circumstances!, clearly