feeding on willow/sallow on pond edge
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I had dismissed this because the picture in my old book was much duller green - but some images on the internet are as bright as this one. It's quite a possibility, as sallow is a food plant.
The Eyed Hawk-moth is a bluish green and the Poplar is more a yellowish green. Colours of pictures on the internet are not always accurate - so many variables can affect the colour of digital images. I have never found Poplar H-m on willows where Poplars are common. The white stripes and colour of the horn point to Eyed I think. On a point of jizz, the Poplar Hawk looks stouter and the Eyed somewhat slimmer.
Lat/Lng: 53.7509, -2.7451
OS grid ref: SD509285