John Browne knows all about the importance of choosing your words carefully.

His reign as chief executive of energy giant BP was brought to a premature end in 2007 after he lied to a court about how he had met his former boyfriend. And then, three years later, the Deepwater Horizon oil spill in the Gulf of Mexico not only raised concerns that he had helped shape a company that put profits before safety, but gave substance to criticism that his "Beyond Petroleum" rebranding of BP had been nothing more than a cynical exercise in "greenwashing".