David Baddiel tries to understand the United Kingdom's constitution. What does a constitution do? Does not having a written constitution mean we don't really have a constitution at all? Should we? And where does the Royal Family fit in? David speaks to Lord Lisvane, a former Clerk of the House of Commons, and to Lord Turnbull, once the country's top civil servant, to understand these and other puzzling questions about the constitution. Producer: Giles Edwards.