Well, that's what it looked like to me. Distinctly darker and duller grey than typical local argenteus Herring Gull, this bird looked like an argentatus from somewhere further North.
This first shot shows the upperpart tones of the 'argentatus' (top left), a Lesser Black-backed Gull (centre) and a typical argenteus Herring Gull (bottom right).
The following shot again shows the difference in the upperpart tone between the two Herring Gulls and a selection of Lesser Black-backed Gulls.
It is difficult to make out here, but P9 on each wing had a thayeri-type pattern.