I live in London, and unfortunatly dont know much in the way of birds. Recently I spotted a black and white Warbler ( looked it up), can anyone tell me if they are often seen out of their native? such a small bird, I dont know much about migration either.
A Black-and-White Warbler would be extremely unusual in the UK: it is an American species, spending the summer in North America. The British Trust for Ornithology page for this species is:
Most of the 13 sightings since the 1950s have been in the autumn, with birds being blown off course during migration - a sighting at this time of year would be even more unusual. Last sighting in the SE of England in the 1990s, although more recent sightings in Scilly and Shetland.
Did you get a picture? News of unusual sightings normally travels pretty fast among ornithologists - we'll check to see if there have been other sightings. If there have not been other reports, it does seem possible that it was another species.
It was 100% a black and white warbler, I looked up the description as soon as I could because it was such an unusual little bird. It didn't seem to be very shy as it was in front of me for about 10 seconds, no bigger then a starling, just off of the Old Kent Road. Unfortunately I didn't get a picture.
Must say that I am intrigued! There is no noise from the birding networks on other sightings, but the Old Kent Road is not usual spotting territory. I had wondered whether you might have been looking at a pied wagtail or perhaps, because it was starling-sized, at a leucistic starling or blackbird (which would have a mixture of black and white feathers but yellowish beaks). Pied flycatchers don't really look much like Black-and-white Warblers so that's unlikely to be a source of confusion.
However, you are very clear about the identification so to take it forward in a scientific way, can I suggest that you consider reporting this to the BTO? You would need to register on Birdtrack and complete an observation record. See http://blx1.bto.org/birdtrack/main/data-home.jsp. There is a page on rare birds as well for more information http://www.bto.org/volunteer-surveys/birdtrack/bird-recording/rare-scarce-birds which tells you how such observations are verified and checked.
You could also phone or email Birdguides, which operates an information network on current spottings - see http://www.birdguides.com/birdnews/submit.asp
I'll look out for other mentions elsewhere.