Cocoa beans are the basic ingredients of chocolate. They are one of the most heavily traded commodities in the world. In Europe, major chocolate makers have signed up to Fairtrade programs, claiming some of their products are made without abusive labour practices. Now the BBC's Paul Kenyon, posing as a cocoa bean buyer, puts those claims to the test, revealing that despite Fairtrade's best efforts unscrupulous cocoa suppliers still try and cheat the system.