I would trim about 30 cm off the top, and feed them lightly with a slow-acting, high nitrogen fertilizer. You might have to wait until spring to do that, since the plant should be starting to harden off for winter, now.

Ah, well you should have pruned them all back by about half when you planted - this forces new growth nearer the base, so they merge and become a hedge. If you live in a milder region of the UK, you can risk doing it now, though I'd suggest only a third off the top to make them all similar heights rather than half, pruning back again (and fertilising) as growth begins next spring.

Long term, remember to keep the top a little narrower than the bottom of the hedge, so it remains bushy to the base.