An old Brown Argus??

[glsammy]

This one almost has me stumped. It was the size of a Brown Argus, i.e. about a quarter of the size of a Common Blue, but there's hardly any under wing markings. You can just make out the feint outline of some dots, so I'm assuming it's a very old worn out Brown Argus. I'm all ready to be corrected!

[Fourwings]

I'd agree with you pal, it looks like a very old/over-mature Brown Argus.
What you can see of the underwing pattern looks correct for an Argus.
An interesting find.