By Lars Mucklejohn Former Barclays chief executive Bob Diamond has called on the lender to be “radical” with its investment bank by either putting more money into the business or selling it. Many investors have urged Barclays to ditch its global investment bank, which is relatively small compared to the dominant Wall Street firms, and focus more on retail operations. “They need to be radical – either invest in it or exit,” Diamond told news and data service GlobalCapital. The investment arm includes part of Lehman Brothers, which Diamond snapped up when it collapsed during the financial crisis…