Strictly speaking, El Jem isn’t a desert town, but as we step off the louage from Monastir and squint through the haze, it bears all the hallmarks of one. We arrive in a dust storm – or maybe a sand storm – which makes the whole town shimmer in the heat and minarets and palm trees drift in and out of sight. Flags whip and crack in the stiff wind, the sun is clogged and filtered by the sand which is so concentrated that it’s a bit like looking into a fog. Our glasses and sunglasses are quickly covered in a film of dust, layers of sand particles have coated…