If the decanter should ever stall it is considered very bad form to ask for it. Instead, you ask the person hogging the decanter: "Do you know the Bishop of Norwich?". If they are au fait with port etiquette they will immediately realise their faux pas and pass along the decanter with an apology. If not, and they answer in the negative, you should say: "He's a terribly good chap, but he always forgets to pass the port."