I've since reviewed the photos and I've come to the conclusion that it could actually be a hybrid and most likely a Herring Gull (argenteus) x Lesser Black-backed Gull (grasselli). Large hybrid Gulls are not an uncommon sight but it's not an area I'm over familiar with so I sought out a couple of opinions and by and large the feedback I received was positive and in agreement that this bird could indeed be a hybrid.
Structurally the bird is identical to an adult argenteus and has a red orbital eye-ring and yellow legs (not strikingly yellow, more of a washed-out yellow). Unfortunately I never got any flight photographs (that's the Med Gulls fault) as it might have been interesting to see the wing pattern.
|Herring Gull (argenteus) x Lesser Black-backed Gull (grasselli).|
To also back up the theory that this bird could be a hybrid, it also appeared to be paired up with a Herring Gull (argenteus) as shown below. This photo also gives you a good impression of the birds size and shape and just how dark the slate-grey tone was when compared with a Herring Gull (argenteus).
As always...comments welcome.
|Argenteus to the left and hybrid to the right|