Spoilers are the last great currency of the world. If they could be converted to hard cash, the global economic crisis could be either solved or worsened, depending on who gleefully leaked details of the new Doctor Who girl or who nobly kept news of the latest Dexter season finale to themselves.

Spoilers, as River Song is well aware, can make or break films, books and – especially – TV series; but what are the rules when it comes to passing on plot details from your favourite show? Is there some kind of etiquette?

You bet your leaked Game of Thrones premiere there is….