A Leylandii hedge can look green, feathery and attractive, but they almost never do. People plant a hedge and then they leave it. Regular pruning is mandatory to keep the hedge looking smart, thick and green, but such maintenance is a lot of work - especially for the person who doesn't own the hedge but lives on the other side of a common boundary. So the hedge ends up like a bunch of individual trees, naked at the bottom and growing out into adjoining land and blocking light and views. There are laws about hedge height - and rights of light etc., - but people spend years and small fortunes in bitter disputes which help no one. Acting aggressively is not the way to go, even if you can't actually see your neighbour for the blasted hedging! As a previous poster has said, throwing (his own hedge clippings) and stuff back onto his property is not a good idea. Better to ask the neighbour directly and politely if they could reduce the height and regardless of the response take the opportunity to say that you are going to have to reduce the huge amount of annual work which this causes you, by cutting the hedge hard back to the boundary on your side. You do of course have the right to do this - as it's on your land. You'll end up with brownish spikes and almost no growth since once the branches are a certain age, they don't recover from hard pruning. Not attractive but can always be screened with honeysuckles, clematis, ramblers etc., planted on your side.Your neighbour keeps his hedge (to maintain) and you have no worry about endless work. Result, I think.