Australia's spectacular Great barrier Reef was a graveyard for shipping, HMS Rattlesnake, an ageing British warship, was commissioned in 1846 to survey   this magnificent 'coral sea' and to produce the first detailed chart of the New Guinea coast. Every reef, every shoal, every rock hazard had to be located and mapped with extreme accuracy. At stake was the pre-eminence of British sea power - and the ambitions of those on board.