Posted: 06/04/2016 at 07:26
To encourage them to grow I'd feed the soil around hedge with Fish Blood and Bone about now and water it well in. Don't trim the tops until they are nearly at the height you want, but trim the sides, shaping the profile of the hedge into a gentle 'A' shape so that it's a bit wider at the bottom than the top. This way the base of the hedge will get plenty of light and stay green and the hedge will be sturdy and not get top heavy.
Trim regularly - never cut back beyond the green of the hedge as it will leave you with brown dead looking patches - Leylandii does not regrow from old wood.
You need to be aware that there is legislation regarding the height of evergreen hedges which may become a nuisance to neighbours. This applies to hedges over 2 metres tall.
Details here https://www.rhs.org.uk/advice/profile?pid=408
Hope that's helpful