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Herring gulls on the right, common gulls on the left.
Hopefully I will soon get to know the difference - and different plumages etc. Cheers
I have found there are a range of things which always help. Leg colour is particularly useful. Also if (like in this picture) there are a few together it is easier to compare the sizes - the Common Gulls are smaller than the Herring Gulls - which UNTIL YOU ARE MORE FAMILIAR can be hard to tell in isolation. Just some thoughts from someone who is probably only a few steps down the road from you on this tricky ID problem.
Feel free to look at my ongoing (hopefully improving) collection of pictures on Flickr:
Lat/Lng: 52.9032, 0.4679
OS grid ref: TF660368