Having done some research, I am now pretty happy my initial identification of this individual was correct, that was of an 'Omissus' type Argentatus Herring Gull, probably a northern bird.
The large bill, coral red orbital ring and rather short winged, lemon yellow legs and heavy impression all support this.
Careful reading of the birds moult was imperative. P8-P10 are clearly older primaries, faded and very worn. Counting from the inner hand, we see that P1-P5 are fresh, newly grown feathers, whilst P6 is currently growing out. P7 has been dropped and is missing. The greater covert's have largely been dropped, exposing the secondaries. Heavy moult is also apparent around the head and face, further accentuating the bill, which appears large and thick based.
I believe with fresher remiges this bird would show a large white tip on P10, with no sub terminal black bar. P9 would show a white primary tip with a black sub terminal bar dividing the tip, the remains of which can just be made out on this birds worn P9. A Yellow-legged Gull would, I believe, show more black on P9 than this bird shows. The next major point of interest is the well defined, sharply demarciated black sub terminal mark on the new P5. This is quite well documented as occurring in Argentatus, particularly in yellow legged 'Omissus' type. In addition I feel the birds general tone on the upperparts to be a little light for Yellow legged Gull.
A rather interesting bird, with P5 in particular leading to niggling doubts about my initial identification. To date this is the most complete subterminal band on P5 that I have seen in Argentatus. However, with careful analysis and weighing up a suite of features, the scales tip firmly in one direction, favouring Argentatus Herring Gull.