By Rhodri Morgan Sales at South African-British diamond merchant De Beers have fallen, the firm reported today, adding to the woes of its owner Anglo American. While Christmas shoppers helped boost diamond sales to $370m during the period December 19 to January 31, this was much lower than the $454m it reported for the same period last year. De Beers chief executive Al Cook said today that “macro economic challenges” had weighed down on the most recent trading figures. “As the prospects for economic growth in many major economies remain uncertain, we expect that it may take some time for rough…