The Lesser Black-backed gull is an Amber species, with some conservation concerns, however, they appear to be on the increase. Although shy, in the last few decades they have followed the Herring Gull in becoming an urban bird. A report by Monaghan and Coulson indicates that pairs have taken to nest on rooftops, with industrial and modern town centre buildings favoured as nesting locations. The first reports of roof nesting in this species are from 1969, and populations nesting in buildings are on the increase in the British Isles.
Patricia Monaghan & Dr. J. C. Coulson (1977) Status of Large Gulls Nesting on Buildings, Bird Study, 24:2, 89-104. pdf here.