Realistically, does this scenario happen often? How many times has someone came across a situation where by the regs it was wrong, but right by the manufacturers instructions?
Surely manufacturers are required to produce their appliances so they fully comply with our regs and not just used as a guide?
I would say though that regarding the flue in this thread. The "standard" would be to clip at every change of direction and just before if leaves the roof. But if the manufacturers produced their flue so that when pieced together it was rigid, and they stated only 1 clip at the exit was needed, then this would be acceptable.