Butterfly Conservation website says of the silver-studded, "Males blue with dark border. Females brown with row of red spots. Undersides brown-grey with black spots, a row of orange spots, and small greenish flecks on outer margin. Males similar to common blue, which lacks greenish spots."
John's butterfly lacks a dark border, so it's more likely to be a common blue. Further, the silver-studded doesn't appear to be found north of North Wales. But both John and I would welcome some help!
Many thanks to John Frame for sending me his pictures.