This 2cy Yellow-legged Gull has been regular at Chiswick Eyot in recent days. It's quite a distinctive bird with heavily marked scapulars and worn flight feathers - the primaries are quite bleached, for example. Covert moult has commenced, as is evident in the pics below.
2cy Yellow-legged Gull, Chiswick Eyot
A large and aggressive bird, here it is pictured bullying a 2cy Herring Gull
I've also had up to three at the Wetlands Centre lately, so they appear to be on the move. This presumably ties in with Lesser Black-backed Gull migration, which also seems to be well underway. Small pulses of birds have been passing through my stretch of the Thames and I had blue-ringed 2cy V.JZ4 last week, which is apparently from Denmark (awaiting details on that one).