All has nothing to do with manufacturer's declarations of conformity
I'd wager Intergas have never made such a declaration, indeed the problem with OV fluing is not covered in the MI's, so their advice is word of mouth only - hush hush, under the carpet, cement it in the wall, customer won't notice etc...
I promise I'm not having a go Dan, far from it. Your choice of words were spot on - 'engineering judgement'. Some use it when taking a chance with CET, others when hiding a push-fit flue joint in the wall.