Great sleuthing everyone. As the hedge has been allowed to get quite unruly I'd like to give it a very significant trim but am concerned about going too far. Is there a general rule of thumb I should use?
Not sure how tall yours have grown but they can take a harsh cut back, but I don't recommend anymore than half of its current height at any one time. If it's a complete over-haul, allow it some time to recover. You could just cut them back in stages throughout the year so it does not look so bare.
I think early spring would be best as well. If you are a cautious type, you could try cutting back one side hard this year, then the other side the following year. If that doesn't appeal, cut the lot back now but as advised above, not more than one-half. If you do it now, then you run the risk of any new shoots being killed by frosts.
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Nor sure....Lonicera pileata leaves are thinner, more pointed and angled forwards.. not seeing that in original pics.see links.
https://www.hedgesdirect.co.uk/acatalog/lonicera_pileata.html
https://www.google.com/search?q=lonicera+pileata+nitida+difference&client=firefox-b&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjfiZvFr8vfAhUHCxoKHYirAX4Q_AUIDigB&biw=1920&bih=944#imgrc=PFF-8i5124S2UM: