To say one thing for John McDonnell, he shows a refreshing preparedness to use a general election to lay out big ideas. While so many candidates for high office will retreat into platitudes rather than risk upsetting some target group of voters, the man who could be Chancellor of the Exchequer in three weeks’ time made a speech on Tuesday signalling what would amount to an even sharper change in Britain’s economic direction than that brought about by Mrs Thatcher’s first election victory in 1979.