Let's take a closer look at a couple of things.
|Juvenile Yellow-legged Gull at Lodmoor. This photo was taken on 18th July. Note the pale pink legs. Although not viewable in the Herring Gull photo, the leg colour of the Ferrybridge bird was identical to nearby juvenile Herring Gulls.|
|Note the scapular pattern of the Ferrybridge bird, with the pale fringes producing a notched effect. This is typical of juvenile Herring Gull.|
|Note the general lack of notching on the scapulars of the Yellow-legged Gull, thus looking a lot neater. Lesser Black-backed Gull is identical in this respect.|
|Like the scapulars, the median and lesser coverts have that pale notching, giving it a marbled look.|
|Yellow-legged Gull, on the other hand, has dark feathers with neat pale fringes. Again, Lesser Black-backed is similar in this respect.|
Now, I know what you are saying, "What about the greater coverts and those tertials."
Well, the greater coverts can look like that in Herring Gull, as for the tertials, well they are unusual for Herring Gull. Remember, and I wish I took some photos to show you, but I didn't, you'll just have to trust me, in flight, the tail and upperwing pattern were typical of Herring Gull. Which leaves the tertials as the only anomalous feature. Well, again, though unusual, Herring Gull can show a tertial pattern like this. Tertials aside, there is nothing else to suggest that this bird is a hybrid. Plumage-wise it is within the variation of that shown by Herring Gull.
As you know, large gulls are extremely variable, which is why you take a holistic approach and identify the bird by a combination of features. Hope this has enlightened you. I know it's certainly bored you to death.