I neither like nor dislike Dave C., and put on the spot I would have taken a wild guess at Elgar too, but for someone who went to Eton, and presumably took Latin, I would have thought that the Magna Carta question was a no-brainer.
Yup, fair do's,Debrajoan.
One would immediately hum the tune in one's head and Elgar would have been my knee-jerk answer too. (Pomp and circumstance and all that.) But the Magna Carta question? Oh dear, oh dear, oh dear.
By the way, did anyone who saw the interview notice how Letterman's chair was positioned about six inches higher than Cameron's? A very old psychological trick and one Michael Parkinson never resorted to.