Hairy-backed Boxhead Wasp Crossocerus megacephalus
I keyed this using the Naturalist's Handbook guide to Solitary Wasps (2nd edition). There were one or two points in the key where I wasn't 100% sure which way to go (e.g. how long is long vs. short when it comes to hairs on the head?), but the end result seems ok I think.
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Hairy-backed Boxhead Wasp Crossocerus megacephalus showing wings, head hairs, pygidium and clypeus, near Gressenhall (Norfolk, UK), 2nd June 2022