I agree with Klaus that - if we are dealing with a hybrid -LBBG involment would seem a bit unlikely and that Common Gull might be a better explanation. A small Herring type doesn´t seem quite likely either given the features shown, such as greenish leggs and primary pattern. However, stranger things has happend! Here
this gul appeard and as can be seen references are given by Brian to the spanish gull here
which is rather similar and a few ears back (if by any chance the same individual), was suggested to be a possible California Gull here
but this was soon rejected by the pale iris to mention one feature. On page 3 in this thread a second opinion was given by Daniel Velasco. Further suggestions of the spanish gull can be seen here
link shows the spanish gull through the years.