This adult
Lesser Black-backed Gull Larus fuscus ssp? was photographed January 23, 2008 near Dix, Oxon, England; it was exceptionally dark - slightly darker than nearby GBBGs; also note the extremely small white apical primary spots - much smaller than on the darkest nearby LBBGs:
In the image below I've cut the target bird and pasted it below the nearest GBBG in the same original image (see below), for comparison:
