Just for your information, none of this is relevant to your actual problem, which is with a piece of tort law called "Land Trespass". Briefly, if your fumes from your flue cross the boundary and interfere with your neighbour's enjoyment of their property then you are at fault unless you have something called an easement permitting your activity, which is unlikely.
You've already admitted you're causing a nuisance by warning your neighbour (see your previous posts) that your fumes maybe entering their property and causing a hazard. At some point, probably when they have difficulty selling their house because of your flue, you are going to get sued. At that point you are going to really regret not spending the few hundred quid it will cost you now to have your flue modified. You may think that plumbers are expensive but that's only because you haven't seen what a barrister charges to tell you you have got no case and need to settle.
I've seen how a very similar case played out in practice. It did not end well for the person in your position, who was just as sure as you that they were in the right.
My advice is to stop getting legal advice from gas fitters and builders and find yourself a lawyer who knows about land trespass and tort.