Azorean Gull at Rainham?

During the four hours I spent at Rainham on Tuesday, I had two birds which, for all intents and purposes, may be hybrids. The first bird, an adult, was the less interesting of the two, and was what I would call a likely Lesser Black-backed x Herring Gull, similar to this bird photographed by Dominic Mitchell in late 2011 (could it perhaps even be the same?).

Adult gull, presumed hybrid Lesser Black-backed x Herring.

The second is a 3cy bird, moulting in to a third-winter plumage. Age aside, it looks remarkably like some of these claimed Azorean Gulls of recent autumn/winters (Oxon, Beds) - mantle colouration at the dark end of Yellow-legged Gull, but also too pale for Lesser Black-backed. Structurally, it felt like a Yellow-legged Gull - big and robust, long-winged, with nice and long yellowish legs. Fascinatingly, it had a broad and full black tail band, with some barring left on the uppertail - again, only just visible in the images but nevertheless significantly more than one would expect to see on a michahellis of this age. The retained secondaries (only just visible in the photos) were a dark chocolate brown, and there was also a brownish wash to some of the new lesser/median coverts. A retained tertial (maybe two, I couldn't see) was dark chocolate-brown with some pale scalloping limited to the tip.

I couldn't make out the eyering colour, but reviewing photos it appears to have some reddish tinge to it at least (although atlantis at this age don't necessarily show an obvious red eyering at this age). The iris is a very pale yellow. The head streaking is dense, creating a hooded effect. I was initially put off by how this streaking extends down the nape/neck, although looking at photos shows that this is not a problem in atlantis at this age - had it been an adult, that might have been different.

NB: have been looking at the primary pattern as closely as my (poor) images will allow; the one thing that concerns me is the pattern on p5 - see eighth photo down - although the black band is complete, it does not seem as broad as in the adults that I've seen shots of in flight. However, it looks pretty similar to the Oxfordshire adult of 2009. There also seems to be a hint of black in the outer web of p4. Unfortunately it doesn't help that the bird is in active moult and the primaries just seem to be all over the place. UPDATE: p5 doesn't seem to be out of range.

I've always been pretty sceptical about Azorean Gulls occurring in Britain - at least in the Midlands - and typically opt for the lazy hybrid option. But this bird has fuelled my interests further - this thing looks almost identical to some of the birds I saw on Corvo last October, as well as sharing plenty of similarities this thing photographed on Sao Miguel in September. It's not far off this bird from Terceira, either.

So, what now? Well, apart from muting it as a candidate Azorean Gull, I don't think you can go much further. It would be great to see photos of known hybrids (Herring x LBB or Yellow-legged x LBB), but they seem difficult to come across. All you can say is this bird shares a lot of features with Azorean Gulls of a similar age. However, it would take a braver man than me to totally rule out the possibility of a hybrid of some description.