You can cut it back hard - but not now, now it can only really have a light trim. Aim to cut it drastically next spring, when growth begins.

when you do cut it back,if you cut every other branch and let them grow new shoots before you trim the rest, (this will not take long )your shrub will still look lush and you don,t get that chop off look,once you have the height you want, just keep cutting back in the same way and your shrub will always look great.

Thank you Bamboo and Taz, excellent news! Thanks for the tips, and I'll wait until spring to prune.

One advantage, though, to cutting it all back and giving it a good feed in spring afterwards, is that when it starts growing, all that growth will be bright red...

Thank you for the extra advice Bamboo. One thing that worries me - and the reason I asked in the first place, but maybe didn't make it so clear - this shrub is now about 10 feet tall, 6 feet wide and 3 feet deep. So as you can imagine, many of the branches are thick, like tree branches. I want to cut it back to about 6 feet tall, and 2 feet deep - the width doesn't matter so will just need a light prune - but when I chop the top and the front, the bare branches that are exposed will be thick ones. Will these sprout new growth? I have another Red Robin in my front garden, just planted last year, and I prune it regularly to keep it the right size for its position, and it only takes about three weeks to have lovely new red growth each time - but then the branches are only young and thin. I'm worried about the much bigger one, being about 10 years old and much thicker branches. I'd be grateful for any further advice or reassurances.

So what you've got is more of a tree than a shrub, currently. If you don't mind it having a thickish trunk and limbs, then just cut right back to those, but if you'd prefer it to be more of a shrub, with less thick wood, I'd be really brave and chop it down to about 2 feet, reducing the length of any branches left coming off the 2 feet of the main trunk. I'd do it in spring which, where you are, is probably earlier than in London, so probably late March early April, depending on the weather. Give it a good feed with a balanced fertiliser immediately, and wait and see what happens. It should be fine and recover well, but even if it's not, they aren't expensive shrubs and they are rapid growers... and you won't have a tree like, thicklimbed plant when what you really want is a shrub like the other one you have.

Thanks Bamboo - don't know if I can be that brave! It gives us a lot of privacy which would be drastically affected if I cut it back so much. I can see what you mean though. I might start off by cutting back to the size I want and seeing what it looks like. I can always continue if I don't like the look of it! Thanks for your help.