Phaonia valida
Before keying this I ran it past the ObsIdentify app and this was 100% sure (with the usual caveats, not least that this isn't in the area covered by the app) that it was Phaonia valida. But I can't trust that (especially given the app does not claim to cover British fauna) so checked the ID by taking it through the keys. Eventually I reached the same conclusion.
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Phaonia valida, North Elmham Cathedral Meadows (Norfolk, UK), 16th September 2020