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I think the colourful eyespots mean it is a male.
I find these very difficult to distinguish from Lang's short-tailed blue - in particular because the tail is not at all conspicuous, in my experience - but I don't think the eyespots on Lang's STB are colourful. (And maybe Lang's STB doesn't occur in your area anyway.)
I think the determinant factor is the wavy stripe which is absent on Langs. We do have Langs here, and I took a few shots of them only yesterday. I have just logged the observation (node/360153) and you can see that the eye spots are fairly colourful.
That's very helpful. I've just put up a photo of what I had identified as Lang's (node/360303) and it doesn't have the broad wavy margin. I can't now remember what made me identify them as Lang's, having first thought they were long-tailed blues, and my book is in Greece not here in London.
Lat/Lng: 42.546820474167, 2.8725814819167
Stony ground by a river.