Line an ovenproof casserole dish or roasting tin (large enough to hold the salt and chicken) with two large pieces of foil. Pour a thin layer of salt into the base, then sit the chicken on top. Cut the garlic in half through the centre, then put both halves into the body cavity of the chicken with one of the rosemary stems. Tear the leaves from the other stems and sprinkle over the chicken.
Pour the remaining salt over the chicken, pulling up the foil to contain the salt in an even thickness around the chicken. Drizzle the measured water over the top, then spread the dampened salt into an even layer over the breast. Seal the edges of the foil tightly.
Bake in a preheated oven, 190°C (375°F), Gas Mark 5, for 2 hours. Loosen the edge of the foil, then lift the package out. Open the foil and crack the salt crust away from the chicken.
To make the ketchup, remove one of the cooked half garlic bulbs from inside the chicken cavity, discarding the papery skins, and put into a food processor with the other ketchup ingredients. Purée until smooth. Brush the salt off the chicken with a pastry brush, then carve normally and serve with the ketchup, salad and warm ciabatta bread.