The shadow chancellor is almost ready to overthrow capitalism, but first he has to give his work experience assistant’s jacket back.

“I put it on by mistake,” John McDonnell explains, wriggling out of the arms and handing it to the tousle-haired youth. “We’re the same size.”

Four years ago it would have been unthinkable that this rebellious backbencher — sacked from the Greater London Council (GLC) by Ken Livingstone in the 1980s for being too radical — could ever have a realistic shot at No 11 Downing Street. But Jeremy Corbyn’s coup and the Brexit-induced meltdown in the Conservative Party have made some form of hard-left Labour government a distinct possibility. For many business leaders, the prospect of McDonnell and Corbyn seizing power is more