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Would be my guess, possibly of the Tenthredo persuasion. I never saw one on the marsh in all the years I visited and worked there, but I gather it has changed much in recent times.
Thanks. Yes, it is indeed changing -for the better, I hope. I haven't seen one of these on the marsh before today, either.
Lat/Lng: 52.3617, -2.2504
OS grid ref: SO830737