Photos were circulated to a few experts on gull identification and the consensus seemed to be that it was a Lesser Black-backed Gull, though one with very minimal head streaking for the time of year.
Goes to show that gull identification, even of adult birds seen from close range in good lighting by a number of experienced birders, is not always easy. A learning experience for sure. Without further ado, here are some photos taken by Barb Charlton and I.
The bird is second from the right. Mantle colour is intermediate between Herring Gull and Great Black-backed Gull, though to me it looks a touch closer to Herring Gull.
Two closer shots by Barb. The bill colour and shape, red orbital ring (hard to see in the photo but was seen in the field), and leg colour is good for both Yellow-legged Gull and Lesser Black-backed Gull. Note the fine streaks that extend down to the chest, a feature that should never exist in Yellow-legged Gull. Streaking should be concentrated around the eye and towards the front of the face, though never extending to the neck and chest.
I took this photo of the bird on the water with a Herring Gull, though they were drifting away from me by the time I walked down closer to the shore. As you can see, the mantle colour is not nearly as dark as on a Lesser Black-backed Gull. At certain angles and lighting situations the mantle appeared darker than this.
This is one of the few flight shots I managed of the bird, though the angle is too steep to see really well. Yellow-legged Gull should have one small window on P10, no windows on the other primaries, extensive black on the primaries P6 to P10, a thin band on P5, and a relatively sharp delineation with the black and the gray.
If anyone has additional comments, they are welcome.