jack52 wrote:
I have leylandii around one side of my property and it is about dead
due to aphids.

Sounds unlikely, how do you know this?

I now want to change to another type of hedge. It is
about 12ft high by 4-5 ft thick, one side is pavement and road and
other is lawn. I use it mainly for privacy so need something that
will give me good thick, quick cover. I am thinking of cutting it
back my side to the central trunk and then planting hedge in between
the roots so I can keep my privacy while the new hedge grows then
after a few years can chop down all leylandii leaving a new hedge
with a decent height.
Can you tell me if this will work and what hedgeing would you
recommend.

Depends on where you are and your soil.

David