FT.com – John Thain says one of the first things he has to do as the new chairman and chief executive of Merrill Lynch is revamp the Wall Street bank’s risk management system. Over at Citigroup (NYSE:C), they have launched a review of risk management processes and UBS (NYSE:UBS) has committed itself to improving its risk control procedures. Quite right too, you might say, given that these banks have warned they face combined writedowns of more than $20bn on their US subprime mortgage-related holdings.