Hello,
This evening I found a nearly summer-plumaged gull for which, in my opinion, "Black-headed X mediterranean" seems the only possible solution.
You can see a (bad) picture of it on:
http://vwgvap.waarnemingen.be/waarneming/view/46491919
I've never seen this kind of hybrid, so if anybody disagrees, please tell me.
All the best
Nico from Belgium