I saw this gull this morning at Lagos fishing port. My brief view was "contra jour" in very bright sunshine and I actually saw less detail than the photos show - it was really just a silhouette. It was bathing in a puddle with Yellow-legged and Lesser Black-backed gulls but appeared considerably larger than both. It was the huge, heavy bill which really made it stand out from the other gulls.
Greater Black-backed Gulls are very scarce vagrants this far south, but could this be a L. marinus (1st winter/2nd c.y.)?