Clignancourt market – or, as it is officially known, Marché aux Puces St-Ouen de Clignancourt, on the northern edge of Paris, in the 18th arrondissement – is the biggest flea market in the world: a seven-hectare junk city. As the American rightly said, taking in this warren of other people's cast-offs – there are 15 to 20 separate markets here, each with a speciality, with around 1,500 shops and stalls in all – could take years. Or it could take a morning or two, if you take the smart Ultratravel route, and hire a private guide.