I am surprised at some of the view being expressed here!
It seems quite simple to me that the boiler was incorrectly installed at the time and the neighbour has just exercised his right to build on his own land.
Of course, its surprising that the OP, any architect and the builder did not notice the problem and mention it at an early stage.
Because I support people taking responsibility for their own problems I would not even advise the OP to make any early offer to pay any of the cost of correcting the neighbour's installation.
Tony
It seems quite simple to me that the boiler was incorrectly installed at the time and the neighbour has just exercised his right to build on his own land.
Of course, its surprising that the OP, any architect and the builder did not notice the problem and mention it at an early stage.
Because I support people taking responsibility for their own problems I would not even advise the OP to make any early offer to pay any of the cost of correcting the neighbour's installation.
Tony