The Safari thinks we got a lifer but what was it?

The Safari didn’t get on to Patch 1 this morning, poor Frank has developed a nasty abscess on his cheek and is definitely feeling under the weather, which incidentally was rather inclement at 06.00 hrs so we didn’t go far.
At Patch 2 it wasn’t as bad as we’d anticipated, the wind was strong but not too cold, which was just as well with what was about to happen.
First of all we saw that the Redshanks were sitting not in their usual runnel to our left but in the one right below us and a tentative peer over the wall gave us a count of 29 without flushing them.
A chap with a GPS aerial and theodolite stick was wandering around on the beach taking measurements; they’ll all be different tomorrow mate! He’d managed to flush everything from our side of the outfall pipe. With the tide low there were a few gulls sat on the far side of it. First we had a look for any more Redshank, just the one, a single Oystercatcher and a Sanderling. We had intended to have a look over the sea as there have been large numbers of Kittiwakes and some Gannets reported recently along with a single Fulmar. But before that we had a quick shuffy through the few gulls present...the last one we came to was star bird of the session/day/year/decade. But what exactly was it?
The following description is taken from the Safari’s notebook – hard to read wot in the notebook as we took the notes with an ungloved hand whilst trying to hold the scope still in the stiff breeze with the other...
2CY gull (1st winter → 2nd summer) very different from others around.
First most noticeable thing was the long weeny narrow bill, almost wader-like. Looked like it had been ‘stretched out of the face with a pair of pliers’.
The bill was quite pointy and not hook tipped, pale sludgy greeny yellow in colour with no real gonydeal angle.
Gonys had a thick dark smudge not reaching the bill tip and becoming paler and weaker towards the face. Didn’t notice anything on the upper mandible.
Eye small and dark but pronounced, almost bulging out of face, head looked to small for thickness of neck and size of body with a very shallow sloping forehead incorporating a long amount of tapering feathering from the eye across top of bill.
Slightly larger than adjacent Herring Gulls could have been effect of the distinctly longer legs which were wishy-washy pink and wide-set.
Head, neck, nape, breast, belly and vent pure white with plenty of ‘nappy’ behind the legs when seen standing side on.
Mantle clean mid grey, a bit darker than ‘argenteus’ Herring Gulls but not as dark as darkest ‘argentatus’ HG’s or Yellow Legged Gulls (neither present for direct comparison). Mantle looked clean didn’t notice any immature feathers.
Coverts were mottled rather than barred with faint pale tips to median coverts and obvious pale tips to greater coverts giving double wing bar effect but only at certain angles. Greater coverts maybe a fraction darker than the medians, certainly not giving the impression of a generally dark brown panel.
A central tertial was fresh, large dark brown centre with narrow pale fringe and sub-terminal anchor mark. Other tertials had smaller paler brown centres with wide creamy white borders. No appreciable tertial step but that could have been down to the attitude of the bird standing facing the strong wind.
Primaries were dark brown with paler tips extending slightly along leading edge, except for P4 (5?) which had a large white(ish?) tip almost touching tertials. Couldn’t see P10 due to viewing angle. Primary projection was long giving the bird an extended tapering rear end.
Brief flight views of the upperside were insufficient to note any detail other than the upper tail looked fairly clean with few dark spots contrasting well with the grey of the mantle and making the dark complete (maybe not at extreme edges?) terminal band with very narrow pale tips stand out. The median coverts were noted as a slightly paler band across the inner wing.
Brief overhead views showed a small translucent panel in the inner primaries extending only a short way towards the feather base (wide flat triangle).
The underwing was generally paler and ‘cleaner’ than nearby ‘argenteus’. The tips to the coverts were darker giving the underwing a somewhat barred appearance. All the outer primaries were dark brown and unmarked.
The body profile in flight was weird in that showed two bulges, fore and aft of the wings, rather than the usual smooth deep breasted profile, this would be consistent with the drooping ‘nappy’ seen on the standing bird.
Behaviour-wise it was aggressive moving the nearby resting Herring Gulls out of its ‘personal space’ by walking up to them with out-stretched neck and open beak with the wings held slightly open though not nearly as much as the ‘typical’ albatross posture regularly shown by CG in pics on the interweb.
Overall a very strikingly different bird the like of which we’ve not seen before. To us it looked reasonably good for a Caspo but were not sure of which, if any, features may give the game way for some hybridisation somewhere in its past.
Distance to bird was 400 yards+, described as viewed through 20x scope except for overhead when watched with naked eye at about 50 foot up. The light conditions were good with thin cloud about an hour after sunrise with the lightcoming over shoulder onto bird and casting no shadows,...Oh for an 800mm lens and 2x converter...and if we tried to draw what we’d seen it would probably look like an oven-ready chicken as our artistic skills are seriously lacking.
Been trawling the usual gull websites but not found anything labelled for mid March that looks the same or very similar to this bird yet.

That said if any one with an artistic bent wants a stab at drawing what we just described feel free to get those coloured pencils out and send us a copy and we'll let you know how close you are to the real thing...between us we might even get a proper ID.
A windy high tide Patch 2 session with duff light conditions gave us about three dozen Kittiwakes careening northwards and a fair number of Common Scoters reasonably close in but impossible to count as they bounced around on the waves.

Where to next? If we take the camera to Patch 2 the mystery gull won't be there, if we don't it will!

In the meantime let us know what the mystery bird in your outback is.