PM suffers humour and history failure in Letterman ordeal Downing Street should know: do not put the Prime Minister on The Late Show with David Letterman. Yet that is where David Cameron was last night, engaged in some friendly back and forth about our nation's glorious past. Two blocks from where The History Boys took Broadway by storm, our own history boy, Eton-educated and all, didn't do so well. Maybe they set it up to coincide with the UN General Assembly, because Boris Johnson was a guest in June? In any event, things weren't looking good when they briefly filled the studio with stage smog in a little joke about the London weather. Then on walked the PM to the strains of Rule Britannia. And so it began... "Do you mind if I ask you a lot of dumb American questions," Mr Letterman begins. Uh-oh muttered the British scribes and Downing Street advisors crammed into the green room back-stage. Question number one: Who composed Rule Britannia? This was just the beginning of the ambush. The Magna Carta questions are still to come. Cameron looks blank. Clearly he doesn't know. "You're testing me now," he said. He got that right, at least. It was awkward, but the PM had a stab. Elgar? Letterman looked dubious and warned that his researchers would be checking. Then he launched into ruminations about the British Empire when a quarter of the globe pink. "Historians would look at that period as just awful," Letterman suggested, before professing to be confused about the composition of the United Kingdom. What's the deal with Wales? Well, it's hard to answer that. But then we turned to the Magna Carta. Well that was "signed in 1215" (good), said Cameron, on an island in the Thames. Letterman thought that bit was wrong. Wasn't it a "big open place" that hadn't been on his tour when he visited London? Finally, the Prime Minster got it. "Runnymede" (which is not an island). But when Letterman asked where the Magna Carta now was, Cameron was stumped. "It does exist," he ventured, he had seen a copy in the Houses of Parliament. But then the final humiliation: What does Magna Carta mean? No clue, blank, empty, without hope. "Oh it would be good if you knew this - we'll find it," Letterman jabbed. Team Briton, we, the inventors of humour allegedly, were left in the dust by the funny American - who'd have thought it? When Letterman asked impishly who would ever have bet against London staging a brilliant Olympics, the reply from Cameron was meant to be Mitt Romney. But the joke appeared to pass him by completely. If Cameron was a US politician, failing a test on the basics of American history, he would be toast this morning. But it raises another question: would the PM pass a citizenship test for his British passport? If the UK version was as strict as its US equivalent, he wouldn't stand a chance. Rule Britannia: Thomas Arne. Magna Carta: "Big Charter."