It seemed like a simple plan for US banks. Find a City stalwart with a close relationship to some of Britain's largest listed companies, and buy it – and get access to its customers.

JP Morgan completed its takeover of Cazenove in 2010 and Jefferies bought Hoare Govett in 2012. But changes in the broking market since then show that even the greatest brands in the City cannot always hang on to increasingly flighty corporates.