Allantus cinctus
At the time I found this I had the key to sawfly genera but hadn't found the RES key to species so was unable to confidently identify it to species level. I no longer have the specimen but looking at it again now, the set of photos I took are sufficient to identify it as Allantus cinctus.
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Allantus cinctus, Burnham Overy dunes (Norfolk, UK), 8th June 2016