Black & brown seabirds Seaford Bay 17 7 2015

I haven't been able to identify these birds. They seem to have black backs and wing uppers and brown undersides. I don't think that they are juvenile Black-backed Gulls. They seemed small compared with the Herring Gulls that were nearby. As you can appreciate, with a bog-standard kit telephoto lens in a fading light, it was difficult to get better shots.Any thoughts anyone?