You can cut these right down and they'll regrow, and you should just about get away with it if you do it immediately, best not done after the longest day - best time would have been May but not later than halfway through June.... Give it some general purpose fertiliser once you've done it (Growmore will do) and keep it watered in dry spells.

I did exactly as Bamboo has said; cut it down to the ground and it is now as big as it was before. I also lop off branches that spoil the shape. As regards extra watering and feeding, I must admit that I didn't do either. Must be very robust and certainly responds to heavy pruning.