Great Black-backed Gull is the world's largest Gull. If you see a big one, they can be monstrous. Larus marinus - the name suggests a pelagic way of life, or the very least coastal, but these days they're a relatively common sight in the right places in London. I used to think there was no way I'd ever see one in Wanstead, and that they didn't stray from the river, but actually I've now seen quite a few, including a couple from the garden. Wanstead lies on the flightpath from the Thames to the Walthamstow and Chingford Reservoirs. Many of east London's gulls roost on these reservoirs - park yourself on Mansfield Hill in Chingford one evening and watch as they come into the William Girling, quite a sight. They return to the reservoirs every evening, and each morning you see them heading back in the direction of the river, no doubt for some tasty rubbish at places like Aveley tip.
I think that they're getting commoner in London. At Canary Wharf the other day I counted at least five in a single scan, and on January 1st in Wanstead, Nick, Tim & I had two - one flying west, and another on the deck near Alexandra Lake. What I once thought was a monster bird (literally) seems to be pretty regular. Today I went for a quick walk around the SSSI at lunchtime. I didn't see many birds, but of note was yet another GBB flopping slowly west. I saw flopping because the wings and manner of the wingbeats, at a distance, reminds me of a distant Grey Heron. Slow, heavy wingbeats, often with significantly bowed wings. You might not think it, but those wings can approach 6ft long! Not quite an Albatross, but getting there. Adults wing tips have large white "mirrors", those of Lesser Black-backeds are much smaller. They're also significantly darker-mantled than Lesser Black-backeds, black, as opposed to slate grey, though unless you see both together this can be hard to judge. As well as the flight action, look for a huge bill. You might think Herring Gulls have large bills - and they do - but this is in a different league - a massive slab. You won't be able to see leg colour on all but an exceptionally close flight view, but on the deck the legs are pink, like Herring Gull.
Very rarely do they seem to linger, they are always coming or going. I've now seen 14 in Wanstead, and of these only three have been on the ground. The Wanstead Bird Reports show 3 for 2009, 8+ for 2010, and, though not yet hot off the press, will show 10 for 2011. Not many, so still a good bird and one to look out for. Winter is the time, ie now, and your best bet is to plonk yourself on Wanstead Flats in the early morning or early afternoon and look for them flying over, as well as scanning the loafing flocks on the playing fields in the morning - you might just get lucky. It can also be very educational, as obviously there are a lot of different species of different ages flying through.