I went to London on 2012 during the end of spring season, and tried to locating some of Banksy’s works. After spent the whole day i managed to find three spot.
The first one found was under the Camden Bridge, the one Banksy vs Robbo graffiti war.
The second one found on the wall of a Poundland shop in London; a sweatshop boy with a sewing machine, this one was pretty new during my visit and i was pretty lucky to find it as it wasn’t that easy to locate this part.
While the last one already erased, the wall should have “One Nation Under CCTV” Post Office yard in Newman Street near Oxford Circus
If you plan a Banksy tour in London, then i suggest you to get the Banksy London book.
And check the route on Google Map the day before.