Dytiscus circumflexus
Most references don't show a vernacular name for this species though some references have it as Wasp Diving Beetle, which vaguely makes sense if you turn it over and only associate "wasp" with the black-and-yellow striped species.
It was the broadly black edges to the sternites that caused me to suspect that this might not be the much commoner Dytiscus marginatus, and the acutely-pointed coxal processes that confirmed my suspicions.
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female Dytiscus circumflexus showing hind coxal processes, North Elmham (Norfolk, UK), 18th March 2024