A sawfly feeding on cow parsley, if I remember correctly
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I can't help with the sawfly, but it's not on cow parsley. I think it's Yarrow (Achillea millefolium), or if not, something else in the daisy family. Not that it's likely to help much with identifying the sawfly!
Could it be Rhogogaster picta? Just a suggestion.
Lat/Lng: 51.4, -0.02
OS grid ref: TQ378686