And the list so far is:
Great crested grebe, on Canada Water
Mute swan, on Surrey Water
Coot, on Surrey Water
Lesser black-backed gull, on a bollard in the Thames
Blue tit (I think -- a great tit would have a black band down its belly) in Russia Dock Woodland
Tufted ducks (male) on Surrey Water
Blackbird (female) in Russia Dock Woodland (terrible photo, which will be updated when I get a decent one)
Stock dove in Russia Dock Woodland
Black-headed gull by Canada Water (they don't have black heads in winter)
Canada goose on (where else) Canada Water
Pied wagtail by Canada Water
Cormorant on Canada Water
At this point, I changed my camera, having bought a shiny new Kodak Easyshare V610 with 10x optical zoom, and also a Gorillapod tripod (since you need steady hands or bright sunlight for 10x zoom). The photos get better, and more worth clicking through on.
Magpies in Stave Hill Ecology Park.
Grey squirrel in Stave Hill Ecology Park
Carrion crow on Mellish Sports Ground
Mallard on Lavender Pond
Moorhen on Lavender Pond
Black-headed gulls by the Thames
Black-headed gulls, common gulls, feral pigeons and stock dove by the Thames
Grey wagtail in Russia Dock Woodland (very blurry, but good enough to identify the species)
Woodpigeon in Russia Dock Woodland
Also seen, but not photographed: robin, great spotted woodpecker, house sparrow.
That makes 24 so far, already beating ajshepherd and armb's votes.