Forty minutes into London Has Fallen, the air is suddenly thick with tension and the promise of conflict. Not from the film itself – believe me – but from the screaming match that has erupted half a dozen rows behind me. A complaint over the volume of a sweet-chomping audience member has escalated into a blazing row, the offender’s mother vocally attacking the manhood of her son’s accuser. Will either get up from their seats? What happens when the lights go up and they meet in the aisle? It was, by a country mile, the most exciting thing to happen…