Leylandii are what they are....Leyland Cypress. Used as a garden hedge they have caused some very acrimonious court cases. A specimen can go to 75 feet. Once they get to a certain height they might cause legal problems on a shared boundary. Coucils have certain powers. Most advice given here is to consider another tree for privacy, if you insist on privacy.

I second that advice.
They are OK if you have a Very Very Large Garden and they are away from the boundaries with other people. Or you have a great passion for pruning and never once miss a session....