We can certainly agree that my question, if it merits anything but a rhetorical answer, has been answered. And quod non certe fits the bill, ita vero.
The gas connection to the new boiler had been made by me but without removing the sealing disc from the recently-installed meter. So far, so legal. The gas connection was finalised by the registered gas engineer yesterday (with my friend present, but I was otherwise engaged) who then fired-up and tested the boiler and the rest of the system. All was in order and the installation was duly certified and will be notified to Building Control by the engineer, who in effect has adopted the work as his own. Job done. (The engineer is a friend of my friend and has certified three or four previous installations done by the pair of us. He therefore knows our work and capabilities.)
Under the old regs, one merely had to be 'competent' to work on gas - and that was effectively a subjective view of the workman (or his critics!). It effectively allowed a self-confident householder to do his own gas work, or indeed to do it as a favour for another. But not to do it for money or in the capacity of (or qua, if you prefer to continue the classical trend) landlord. The new regs are stricter and effectively require a recognised qualification; the requirement to notify work to BC providing the principal check on who does what. But this does not of course prevent an unqualified but 'competent' person from meddling with gas in situations where (a) no BC record is required (which precludes a new installation or most appliance changes); and (b) no unwanted physical consequences arise. The comment by the BC officer you quote was therefore historically valid (if glib), except that the key word should be 'competent', not 'qualified', but is no longer. And the person who causes an explosion might well be demonstrably 'qualified' (and therefore considered competent) but on that occasion made a mistake which threw doubt on his continuing competence. (I am qualified in two professions but that does not of course make me incapable of making a mistake when following them, of whatever degree of seriousness.)
As to the 'boiler fixing', the only way I could envisage a 'top anchor' to serve the same purpose as the impossible vertical screw would be to pre-fix a small angle bracket to the boiler case top and screw horizontally through this into the wall behind, using a good light and a long screwdriver (or socket). But I don't think that would be possible withour piercing Mr Glow-worm's lovely casing, probably invalidating every warranty as well as showing a degree of inventiveness and common-sense that Mr G would deprecate. I'm sure you are absolutely right about the tick in the box. As it stands (or hangs), we have done nothing to substitute for the screw and the commissioning engineer was quite happy with this, whilst sharing our surprise/annoyance at the design fault.