Re: Duel fuel problem.
On going Gas Safe registration prevents a registered gas safe engineer signing off of or accepting as his own, another engineers work unless under very very specific conditions, (limited ability if training an engineer whom has passed his basis training at a suitable registered college for the purposes of his training) which from the nature of your post will not be the case.
Sorry but if it were me, I wouldnt chance it.
Regards
Jackal