Why not approach the neighbours first, i.e. now? If they're sympathetic
and actually do this, you gain two ways (if you end up buying the
house). If they're not, you still gain two ways: (a) you find out in
advance that you'd have had a problem and (b) you avoid some
unsympathetic neighbours.

I can testify myself (being both on the receiving end and on the
delivery end) that it's *very easy* to allow a leylandii hedge to grow
taller than you ever intended: the bigger they get, the faster they
grow.

John