Confidence: It's likely to be this, but I can't be certain.
Notes: Compared to the most likely confusion species (Black-headed Gull) the black head spot is slightly larger. Legs pinkish and lighter than those of the neighbouring BH Gulls. Distinct narrow trailing black line on wings in flight. Underside of wings seemed much cleaner than on BH Gull. Dark markings on outer primary coverts. Black bill.