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This is a tricky one, but I think I agree with Michael. There is a lot of black on the forewing, but the summer generation of the SW has a fair amount of black on the forewing. It does look like quite a faded individual but, even so, the black on the edge of the forewing doesn't seem to be extensive enough for the LW.
Yes, I was a bit hasty on this one. It seems I can't modify an ID once made, so its "likely ID" will have to stay erroneous until a few other people ID it as small white.
Thanks for all the feedback,yes definitely a tricky one!
Lat/Lng: 52.1, -3.4
OS grid ref: SO0451