In my view it is a brown argus.
Check out the female common blues here
Notice the orange lunules on the hind wing are next to black spots with a white background and also the female has a blue flush to some degree. Your specimen has no hint of white near the base of the hind-wing orange lunules spots and no blue flush.
Thanks for that, I really appreciate it.
My book shows some female Common Blues without any hint of blue whatsoever.
I take your point about the orange lunules and the hint of white.
My thanks to all who took the trouble to reply.
I saw the Brown Argus on a butterfly walk organised by the Suffolk Wildlife Trust today at Nettishall Heath, between Thetford and Diss.