I have not seen L. f. fuscus for a very very long time (30+ years). According to Olsen & Larsson ‘ Gulls of Europe, Asia and N. America’ , adult winter L. f. fuscus has the upperwing black with NO contrasting wing-tip (the photo shows a distinct contrast).
2nd Winter L. f. fuscus should have strong head markings around the eyes and on the back of the head, bill should have black subterminal markings and coverts brownish tinged.
It is a pity there was no photograph of the bird flying away.
Glamorgan Bird Club 
Thanks, I made the mistake of seeing the mirror only on p10 and the general darkness of the bird. Having read a bit further in Olsen I see that half of british LBBG will also have a mirror just on p10. Live and learn