Gymnocheta viridis
This individual was the first one I'd ever fully keyed out to make sure the ID was correct.
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Gymnocheta viridis, North Elmham Cathedral Meadows (Norfolk, UK), 17th April 2021
Although this one wasn't keyed, I think there's enough visible in the photo to identify it as this, at least when factoring in the time of year (the similar Chrysocosmius auratus is apparently a summer-flying species).
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Gymnocheta viridis, North Elmham Cathedral Meadows (Norfolk, UK), 29th March 2019
This one was retained for checking after photographing it in the field.
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Gymnocheta viridis, Dillington Carr (Norfolk, UK), 11th April 2022