F ifty years ago this December, cinema’s greatest underdog story hit the screen, a tale about a Philadelphia ham-and-egger who takes the world heavyweight boxing champ all the way to the final bell, losing the fight on a judge’s decision but winning the hearts of a nation. That film, of course, was Rocky. Its creation, famously, was no less remarkable an underdog story – one about an out-of-work character actor with the curious name of Sylvester Stallone who wrote his own boxing movie then managed to convince a studio not only to finance it, but also to cast him in the title role. He then went one further than the character he was playing by earning the top prize: a best picture Oscar.

If you thought, half a century on, Rocky’s punching power – its unerring ability to make heroes out of zeroes – has faded, think again.…