The echoes seem to reverberate louder and longer when you make a gaffe at Blenheim Palace, the vast and assertive residence of the Dukes of Marlborough, as well as birthplace and burial site of Winston Churchill. Fifteen years ago, on holiday nearby, in a house borrowed from a friend, I paid ten quid or so to admit two to a guided tour of this awesome monument to an England well on its way into the Dreamtime. With its central buildings and courtyards, covering nearly two hectares and containing 320 rooms, including a double-domed library not much smaller than the gentleman’s…