The Chalkhill Blue Butterfly (Polyommatus cordon)
The prettiest (I think) of the Blue species in UK which can only be found in somewhat limited, but well established locations mostly in Southern England. One brood only occurs between July and September. The species is limited in habitat due to its preference for warm chalk or limestone hillsides. Knapweed and Field Scabious are its favourite flowers and the first place to look to find it.
The female rarely perches with its wings open so I was fortunate to obtain these images.