On 20/03/2016 10:59, Steph wrote:
I have a row of Laylandii planted last year, so far they are 3-4' high.
They were over run with weeds last summer and I sprayed carefully around
them with Round Up weed killer or so I thought. Some appears to have got
on one of the trees and the top half has gone brown and looks dead.

What's the best thing to do, cut off the brown or wait for new growth
from the brown stem?

TIA.
You will be extremely fortunate if they sprout green from the brown. One
of the problems with leylandii is they do not regrow on old wood. As
they are prone to dying off in cold winds they are often unsightly.