I was at Dungeness in late December 2007. Whilst I was there I saw this bird on the beach at the Patch. It certainly did not look like any of the other Caspian Gulls I had seen at Dungeness earlier that morning and earlier in the week.
A photo of the bird can be found at the following link: http://www.flickr.com/photos/23555819@N07/2246999769/
I have also have a link to a more typical Caspian Gull seen a day or two earlier
I thought it might be a hybrid due to it seemingly showing features of both Caspian and Herring. The bill was long (appears shorter in photo due to the bird looking slightly towards me), and there was hardly any gonydeal angle.
Structurally, the bird seems to show the protruding breast of Caspian, and rounded head. However, the bird seems to show a prominent "hanging belly" behind the legs (which I thought was more of a feature of Herring Gull). The bird also has the small black eye of Caspian Gull. The streaking on the head also seems intermediate between Caspian and Herring. It shows slightly more concentrated streaking on the nape (like Caspian should) but instead of showing a nearly all white head (as in all the other adult Caspians I have seen) it shows faint dark markings over much of the head (perhaps revealing the slight Herring Gull influence). Another feature this bird shows which I have noticed on Caspian are the white "eyelids". The legs are long and thin, however (and this is one of my main problems with it being a Caspian) the legs are a bubblegum pink colour, very reminiscent of Herring and not at all like any Caspians I have seen. Also the bird does not seem to have a pronounced tertial step (Does this support Caspian?).
The mantle colour is lighter than most of the Caspians I have seen in real life, but from photos I have seen it looks within the range of Caspian. The colour is certainly greyer than the presumed argentatus Herring on its right and lacks the bluish tones of that species.
Unfortunately this is the only photo I got of the bird and I did not manage to see it in flight. I have never seen a Caspian hybrid in the field before so if any of the above features I have used to reach my conclusion are wrong or there are other features that I have overlooked, I would very much like to hear about them.