dellzeqq said:and digging them out is hard work. Why not cut down and dig out two in three and plant something in between? Then, when the hawthorn or beech or whatever has grown to a few feet you can get rid of the rest.
Ground is llikely to be dry and denuded of goodness below leylandii - much better to remove completly and feed the ground before replanting.
Cutting to a 5 ft stump and rocking out using a mattock to cut the roots is not "too" hard - a few mates and a case of beer is recommended!