I am not sure if it is this as the book says there is also S.selenitica but I am not knowledgeable enough to tell the difference
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Nice sequence Sheila showing all the right pointers
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selenitica is a similar large hoverfly with the same swollen forehead, but it has yellow bars (not white), which are not angled so obliquely as those of pyrastri. It is far less common. I found one, though, this morning, on Cat's-ear on a local heath.
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Thanks for the comment's and for the information so hopefully if I find the other I will be able to tell the difference.
Lat/Lng: 55.0571, -2.2324
OS grid ref: NY852736