The historic decision by Standard & Poor's to downgrade US government debt over the weekend has shown up the increasing divergence between ratings agencies' approaches to judging sovereign debt, in a more volatile market environment.

Three years ago, before Lehman Brothers went bust and the world was a calmer place, many western nations' credit quality was unquestioned. According to Chris Bullock, a credit portfolio manager at Henderson Global Investors: "In 2009, Ireland was rated AAA by all three agencies; Greece was also AAA just a few years ago."