october is thought to be the best month to plant trees because theres still enough warmth in the soil and strength in the sunlight to allow some root growth before dormancy sets in...i would wait until after the second week of february now that winter is almost upon us...with newly planted evergreens depending on how hard the winter is you can lose them due to frozen soil ,the plants not having shed leaves still lose water but are unable to uptake from frozen ground.

yes you do get fast growth from leylandii but would nt it be nicer to use native plants?...have you considered Holly if the wind-break needs to be evergreen..??