Interesting Gull at Shawell A5 Lagoons

I saw an interesting gull last Wednesday and again yesterday at Shawell A5 Lagoons. Initially I thought it was a Yellow-legged Gull due to its grey mantle being slightly darker than the nearby Herring Gulls. However, something didn't seem right as its sleek body and long wings gave it more of a Caspian Gull look, but the bill seemed too bulky and bright for that species.

Interesting Gull

The bird spent much of its time doing very little apart from resting on the shore. 

Its legs were a dull yellow or mustard colour and quite long and spindly; its eyes were dark and its plumage seemed to match Caspian Gull apart from perhaps the mantle colour being a bit too dark. The greater coverts were dark centred creating an obvious dark bar

At times the bill seemed very long and slender, but at other times it appeared quite bulky. The bill does seem to be too yellowy for a Caspian Gull of this age.

Please watch the video below.

I had a brief view of it in flight and the underwing seemed to be a little too dark for a Caspian Gull more like YLG.

I'm not sure at present what species its parents were, but it would seem most likely that  YLG and Caspian Gull were involved. 

Feel free to let me know what you think.

Spindly Legs