Caterpillar infestation!

Hi - I’ve just come back from my mums in Blackheath and both her privet hedges have been totally destroyed by these caterpillars. Hundreds of them. They’re not the toxic one’s spoken of in the press I don’t think, because they’re not super hairy. They haven’t eaten anything else in the garden, but managed to totally decimate two 6ft wide hedges so much that it’s gone brown and looks almost dead. You can also see this horrible carpet of webbing all over the hedge, which I guess is them getting ready to turn into chrysalises. She didn’t want to spray the hedge just in case the birds then ate the caterpillars and it killed them. However, the birds haven’t touched them anyway - I thought birds ate caterpillars? Maybe they are poisonous after all?. Anybody got these critters in their hedges around here and know what they are and what to do? Have a look at this creepy crawly and all the cobwebs around it!

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Just found out what it is from this article from last year. My mum’s box hedge now looks just like this one in the article!
https://www.ebts.org/2017/08/chiswick-box-tree-caterpillar-invasion/