Leylandii will carry on growing upwards after having the top cut out. They will recruit new leaders. If you only want one leader you can cut the others out, but do so fairly soon (eg a year later) for reasons that will become apparent.

Unlike broadleaf trees, leylandii won't thicken up as a result of top pruning. Spindliness is a result of shortage of light, or youth. Very young spindly trees will thicken up naturally, without any pruning assistance. Leylandii never grow new shoots from bare trunk or bare branches, ie, inside the zone where green fronds are produced.

If you don't want your leylandii to grow outwards too much, you must prune them every year, but always ensuring you don't cut inwards of the frond zone. If you cut inwards of the frond zone, they will remain permanently bare, because new fronds and new shoots will not grow on bare wood.

Because of these curious behaviours, leylandii are the wrong tree for most purposes. There are many other evergreen trees that are much easier to manage. The only good feature of leylandii is that if you prune them at ground level, they don't grow back. Though actually the 140 ft specimen leylandii at Bedgebury Arboretum are magnificent trees.