Rip it out and start again with beech. The leaves die but stay on so you still get cover.
You'll need to dig in plenty of good quality mulch to refresh the soil and make sure you water id regulalry through the first summer.
Go to:
ReadyHedge : Hedging UK : Ready Spaced, Ready To Plant, Ready When You Are
You get get instant hedging up to 6 feet high. It'd dead easy.
For something different there's an evergreen called Photinia Red Robin that is green but new leaves are red and this is really attractive too.
Don't plant leylandii!!