I suggest you think about legal aspects of tall growths on a boundary......especially Leylandii. Can you keep control Opel once they get going. It can lead to neighbour problems too, If you shade their garden. We will have to pay tree surgeons soon to see to a hazel tree and a laburnum. They shade us not our neighbours. I wonder how near to your boundary is this house and will you be really overlooked? You can also ask your self ....Does it really matter if someone can see.into your garden?