I thought this was a Common Blue, but on closer inspection, I think it might actually be a Silver Studded Blue, which would be rather exciting. The key to identification, apparently, is the presence of silvery, light reflecting scales on the underside of the wings. They are slight, but they’re there. I’m awaiting conformation of the ID though.
Updated to add; yes they are Silver Studded Blues! One of the UK’s rarest butterflies. A more comprehensive post about them is here.