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Cut them back as hard as you like. Now is the best time to do it. They will regenerate. It will take two years for them to flower again.
They may have spread laterally via rooted layers. If possible dig / pull these out.
Thank you for your reply, Gold1locks. It has given me confidence to be really drastic. (There isn't any other way, really! I will post before and after photos. It's going to be a big job, but I can't wait for the space it will leave me for new plants. I'm going to try and leave some growth which will hopefully grow to create a manageable barrier.
At my old house we had 2 huge ones which for similar reasons to you I didn't want to remove.
Read everything I could and they said about rejuvenation pruning by cutting out 1/3 of the mature branches.
I did it the opposite way - Cut out 2/3 of the mature branches. It meant I still had some height for the privacy. It was slow sprouting again but I think after about the third year I could have cut back the final third. In act I didn't as I found I had a layered effect which I liked
It did look a bit sad for a year though (bare)