There wont be a way to claim ownership of 4 or 5 inches of a border line. Where there is a wall or fence is where the line is drawn, even if originally the line was a few inches into the other garden. If the other garden owner put a fence or wall up with a gap then the border line is central between those walls. In other words a border can move slightly by what is built along that line, it is not measured accurately enough to claim ownership of inches either way.
So unless your neighbour puts up a new fence or wall giving that gap you need, you wont have the right to have a flue into that direction.
If there was a fence and a gap between, that the neighbour removed, he does not have to rebuild it.
Hope that helps