Dave,
Howdy.
Thanks very much. I had wondered about the top picture being a Brown Argus Butterfly but plumped for a female Common Blue; I'm not aware of Brown Arguses being seen on the Erith Marshes which is where I saw the Butterfly, so it would be really super-dooper if it is a Brown Argus.
Please see another picture of the Butterfly:
The Butterfly has a noticeable blue sheen. It also has fairly prominent dark spots in the centre of the forewing which, as I understand it, could suggest Brown Argus, although I have seen pictures of female Common Blue Butterflies that appear to have similar markings.
And I have been looking at the abdomen; which seems quite long compared to the, as I understand it, generally shorter version on the female Common Blue Butterfly.
Kind regards ~ Mike ...