Unlike any other public space in London, Trafalgar Square has been protected from commercialism, without advertising billboards or neon lights. But that's about to change. The famous St Martin-in-the-Fields church is to get a giant poster site. It is being done for a good cause, to help fund the restoration of the famous St Martin-in-the-Fields church, but will it lead to an eventual avalanche of advertising in one of London's most historic areas?