Plebejus argyrognomon - Reverdin's Blue
I have seen this species only once, but must make a point of re-visiting it in the future. I recall it's shade of blue was somewhat paler than the other Western European Plebejus species, but that is on a tiny sample size.
It still holds a charm for me in terms of being one of my favourite butterfly photographs.
The orange lunules can plainly be seen to form a relatively parallel band, both fore and hind undersides, a distinguishing feature.