COMMON GULL with resemblance to RING-BILLED GULL at Grovehill Playing Fields

Crop of head detail of COMMON GULL - image enhanced to counter fog.

Going back through some old record shots I came across some of an interesting gull I noticed one foggy morning at Grovehill Playing Fields on my way to work on 12/03/14. I'd stopped off to look for the Mediterranean Gull seen two days before. I noticed at the time this COMMON GULL looked superficially like a Ring-billed Gull (thicker bill, more obvious bill ring, paler eye, stronger build & less friendly face) but the primary wing feathers appeared as they would for a regular Common Gull. I was in a rush so I quickly took some snap shots. Looking at the images, due to a combination of subtle differences to the 'norm' in its features, the bird looks very similar to possible Ring-billed Gull x Common Gull Hybrids, which there are only very few confirmed instances of worldwide.

So, although this is just a particularly well built individual & variant of Common Gull, not the usual dainty & petite look, perhaps these photos & observations will be of use with sightings of similar birds, & unsure Ring-billed Gull IDs, going forward.

[Barry Reed kindly sent me this link which shows a similar bird (near the bottom under 'Unusual Common Gulls'). It has the same colour eyes & similar bill details (though not as thick near the tip, as steeply sloped up from the tip, or as long) & I notice the author also considered the possibility of Common Gull x Ring-biled Gull Hybrid: http://www.birdersplayground.co.uk/Common%20Gull%20-%20adults.html]

Here are the shots I took - some completely unaltered (apart from cropping), & some others enhanced to counter the dulling effect of the fog. You can click on the images to enlarge them.

COMMON GULL - image enhanced to counter fog.

This is a summary of my observations:-

BILL

Generally thicker & stronger than usual for Common Gull, though not as much as I'd expect for ring-billed Gull.
Longer than most Common Gulls.
More steeply curved up from the tip than usual in Common Gull.
Thicker near the tip than most Common Gulls.
Blackish bill-ring which was more obvious, darker & thicker than similarly plumaged Common Gulls nearby.
Thicker lower mandible usual Common Gull - in fact 1/3 - 1/2 way from tip lower mandible is thicker than upper mandible which Ring-billed Gull can show but generally not seen in Common Gull.
Top of upper mandible straight to slightly convex - always seems to be slightly concave usually in Common Gull.
Trace of reddish skin at gape.

EYE

Less 'kind / soft / pretty' appearance than usual Common Gull.
Lighter yellow iris than expected for Common Gull (obvious even when viewed through fog), though not as light as I'd expect for Ring-billed Gull.
Red eye-ring.
Higher-up on head than usual in Common Gull.

BUILD