See the London Cycling Campaign page on bicycle security and download the information booklet. LCC also does cycle theft insurance for its members.
Brick Lane Market
The LCC group in Tower Hamlets have a good website with information on stolen bikes and have begun a letter-writing campaign to combat the illegal trade in stolen bikes in Brick Lane. Don’t forget thieves operate all over London so this problem will almost certainly affect you!
Stolen Bike Index
The Stolen Bike Index is a UK based not-for-profit site that aims to provide advice on avoiding bicycle theft, to reunite people with their bikes, and to help the trade avoid buying from thieves.
It features a reporting system for stolen bikes which can be received by email alerts using the stolenbike-uk Yahoo group. You don’t have to join however to check the message archive of stolen bike alerts.