Thanks, Smiths. I suspect a hybrid is more likely, but I did have a look at Azorean Yellow Legged Gull which is smaller than Herring Gull - wasn't sure about other YLG races though. I knew that Yellow Legged Gull is usually quite a big gull, but not all races are. "Omissus" Herring Gull has also been suggested as a bird to be considered - and the mantle shade is perhaps closer to Argentatus HG than to LBBG, though this bird was more obvious than the Argentatus HG that show up at a local pond in winter.
Nutcracker - Thanks. I deliberately left the exposure(s) unaltered, so that all photos were taken under the same conditions. It was very overcast and rather gloomy at the time.
Laurie - if it is a LBBG then it is a very pale individual -as can be seen compared to the LBBG that was nearby in the other pic. I did find 1 photo on Gull Research Organisation webpage of a LBBG that had a broadly similar shade in September - though photographed in strong sunlight in Portugal (feathers bleached by sun?). Despite Dundee being Scotland's sunniest city, I don't think we get strong enough sunshine in this area to bleach LBBG feathers to the same extent as they get in Portugal.
Pic 1 is perhaps a better photo than the originally posted ones, and the 2nd pic is the nearby Lesser Black Backed Gull showing the difference in mantle shade.