I originally concluded that the 4th bird from the left was a Lesser Black-backed Gull as evidenced by its yellow legs -quite bright in person but a bit more dull in this photo. The size and bill were troubling me, however, (too much like a GBBG - could it be a hybrid?) and so queried some more knowledgeable birders each of whom concluded that this was a Great Black-backed Gull with abnormal leg coloring. Their feeling was that in every other way it appeared to be a GBBG. So... A GBBG can have yellow legs! Who knew!

I subsequently looked for other examples on the web and found these two: http://www.flickr.com/photos/little-w/6140347793/ and http://www.talk.gull-research.org/viewtopic.php?f=6&t=149

For those interested additional photos of this bird are in a small gallery at http://www.pbase.com/rickbowes/2013_08_18_dbch_lbbg