Story : Political commentator Andrew Marr assesses what it takes to be a successful British premier based on the performance of the twenty prime ministers of the 20th century. Advocates such as Martin Bell, Helena Kennedy, Simon Schama and the late Bill Deedes champion their favourite PM in short films, while Andrew and a panel of historians and journalists debate what qualities they brought to the role and how successfully they did it, before coming to a decision on who was the greatest.