very small, just over a centimeter
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Looks more like a brindled to me, by looking at pictures of both.:)
It is all very well comparing it with pictures, but which pictures? Pugs are difficult enough in the flesh. I simply don't see the characteristic dash marks on this photo to make put it in the Brindled/Oak-tree camp.
I'm inclined to agree that it doesn't look like Brindled Pug, due to the lack of dark dashes and the relatively prominent white tornal spots on both wings. Could be an early Common Pug but I'm not sure!
I'm also inclined to agree that pugs are hard to identify ...
Entomologist and biological recorder
Lat/Lng: 51.2214, -1.4703
OS grid ref: SU370470