Looks like a slightly inflated hind femur, but I don't think I've seen this one before.
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Thanks Ophrys - very interesting!
I would say that if the locality contained Petasites hybridus then it could be N. obliqua, but in general in southern England this is not a common occurrence and it is likely to be N. podagrica
I don't know of any Petasites in the area, so podagrica sounds definitely the more likely, although it remains undetermined,
Lat/Lng: 51.2157, 0.3437
OS grid ref: TQ637488