I was with a mate last year who was having his inspection by GS, he took them to a job we'd done on a Vaillant install. Sand n cement, inside and outside both covers fitted.
GS bloke pointed out that we hadn't put the three screws in the internal white flue cover, I said it's been stuck back with a bit of silicone to keep it in place, he said ''If there are screw holes in the cover, then screws must be inserted and screwed into the wall' I was a bit gobsmacked tbh considering some of the sh1te I drive past every week.
Edited to add : I always core 5" btw.