Happy birthday to The Entertainer. The ultimate state-of-the-nation play is 50 years old. I’ve never quite bought the idea that Archie Rice, a failed music-hall comedian, is supposed to represent Britain’s decline as a superpower. A clapped-out comic to symbolise the death of a military hegemony? Don’t get it. But at the time this revolutionary play fomented a new kind of ambition for the theatre.