Looking it up in the book it's what I suspected it was; a holly blue. The common blue is more strongly marked on the undersurface of the wings, with a number of orange spots. There's no orange on the holly blue.
The individual we saw was a male; the female holly blue (right) has dark dark margins to the blue areas of the upper wings.
Summertime BluesWriting in Podiceps, the newsletter of Wakefield Naturalists' Society, in September 1991 Roger Gaynor noted;
Holly Blues have mixed with the boys in blue near the police headquarters in the centre of Wakefield. They have also been seen around Crigglestone, Horbury, Sandal and Oulton. This appears to be part of a national expansion northwards, with large numbers also being recorded this year at Spurn Point and Danes Dyke near Flamborough Head. At one time this species was very limited in Yorkshire with the largest colonies being recorded in the Nidderdale area by Harrogate Naturalists' Society. A similar colour scheme has also been maintained by several broods of Common Blues at Bretton Old Brick Works, and very large colonies have been noted at Heath Common and the new Southern Washlands Nature Reserve.