You cannot discharge a boiler flue across a neighbouring property boundary where it will cause a nuisance, regardless of relevant clearances to openings in buildings.
A fanned draught flued boiler has a clearance of 300mm horizontally to an opening, or 300mm directly below an opening. Also the same measurement for internal / extrnal corners, and 600mm from a surface facing the terminal.
Unless the manufacturers instructions are different then you observe the clearances stated in them.
Aprroved Document J (section 3.23 re nuisance from flues)
Footflaps – those instructions do not relate to a gas boiler, different fuels have different regulations and go about things in a different way.
Mail me if you need more info timATj-twren.eclipse.co.uk