By Mark Kleinman Mark Kleinman is Sky News’ City Editor and is the man who gets the City talking in his weekly City A.M. column. This week he tackles business investment, shares in NatWest Group and Guy Hands’ pursuit of Amber Taverns If Britain is open for business, prove itSo that’s one election campaign commitment broken. Labour said during the run-up to July 4 that it would hold a major investment summit within 100 days of coming to power. Its October 14 jamboree will, therefore, just miss the cut. That’s a triviality, of course, if Sir Keir Starmer’s administration manages to cobble toget…