I hastily took some images of it whilst it bathed and these show a set of features consistent with it being an unquestionable Caspian Gull.
|Bathing Caspian Gull|
The photo above clearly shows the primary pattern associated with Caspian Gull. A long white tip to P10, a large white mirror on P9, a relatively small amount of black across the primaries and grey tongues which eat into the black. The narrow band on P5 is not visible in this image as the feather is hidden, but it is visible in some of the other photos. Also note the dark eyes, which on close inspection are coffee coloured. Its legs legs are pale yellow.
Having spent as much time as I have spent watching gulls in the last two years, it has become obvious that each individual gull is slightly different. It is important to keep an open mind as field guides are really helpful, but they only give an impression of what to look for. Without the photos I might well have let this one go, as the length of the observation was short and on first impression it wasn't a classic. Nothing to me suggests its a hybrid. Female Caspian Gulls are said to have weaker and shorter bills and rounded heads so this probably explains why it didn't at first glance appear to be a classic individual.