The Gull From Atlantis?

On September 29th I saw and photographed an interesting gull at Shawell A5 lagoons. I immediately became interested due to the extensive streaking on it's head. This was far more extensive than on any of the 60+ Yellow-legged Gulls (Larus michahellis) that  I've seen this autumn at the site. The mantle colour was also interesting as it appeared quite dark. You can get an idea of the mantle colour from the photo below, which gives a good comparison with Lesser Black-backed Gull. Could this bird be an Azorian Yellow-legged Gull (Larus michahellis antlantis)? 

I have asked a few people for opinions and so far the replies have been:

'That gull looks rather dark for a Yellow-legged but of course Azorean YL is dark. The head streaking is not as extensive as on a typical Azorean - I think it should extend further down.'

'That is a very interesting bird. Like you say you don't see michahellis with that density of head streaking or with that hooded effect.. I've probably seen upwards of 200 YLG's (probably more) on lagoon 4 since mid August (still saw 23 a few evenings ago) and just recently several very well marked adults have been present but they don't match your bird. The facial expression of your bird is also the same as the RW bird. It's hard to put into words but the pale eye and streaking gives atlantis birds a different expression to michahellis. The sharpness to the edge of the hood on your bird is also nothing like michahellis. The wing pattern looks identical to the RW bird with just a mirror on P10 and the black cutting across the tip. You do sometimes see michahellis with this wing pattern but not in combination with a hood and dark upper parts.  Lots of people try to write these birds off as hybrids but you simply can't when they look like what they should look like. Your bird does have a more restricted hood but the variation shown among argenteus, argentatus, michahellis and Caspian will almost certainly extend to atlantis as well.'

I have not seen the bird since and so have not been able to share it with others. Also due to the difficulty of getting a positive identification I had not widely discussed the bird until now. A third experienced gull watcher has also become excited by this bird and compared it to an Azorean type seen in Hyde Park during 2006.

I would be pleased to get any opinions / help with this bird, as I'm not happy to just write it off as an unusual michahellis. The head streaking may not be extensive enough for some people, but anyone who watches gull regularly knows that the extent of head streaking can vary dramatically amongst gulls of the same species.