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Taken in York a week ago, anyone any ideas, thanks for looking

The blue suffusion round the body does enough to convince me it's a female Common Blue, Charlie, although the characteristics you've marked on your image are relevant as well.

Someone assured me it was a Brown Argus but I wasn't convinced, so thanks David for clarifying the situation.

These common blue/brown argus pics are normally pretty clear cut for me but this is a tricky one. I'm leaning towards Davids answer but would love to see another picture from a different angle to be sure, I'm presuming this is the one shot. Even a blurry one of the underside would go a long way for confirmation. From what I can make out the abdomen looks quite chubby, so I'm pretty sure it's a female of whatever species.

It doesn't look like a brown argus to me. To be honest, if someone had sent me this photo as an e-mail (I get a lot of ID e-mails) without any other info, I would probably have replied, 'Chalkhill blue, female', and then carried on with my supper. But given the fact it was taken in York, I think it has to be common blue. So I dodged a bullet there!

In the field, female common blue is completely different from female brown argus (I agree with Buggy that this is a female). The latter is small and distinctly silvery in flight - an effect produced by the lack of any blue on the wings, so the dark upperside contrasts with the pale underside. Female brown arguses are also very rounded - the wing shape in the present insect looks wrong. I can't rule anything out on one photo, but I would have little doubt this is a common blue.

Guy