Heat the olive oil in a large pan, add the garlic and pepper flakes, and cook for 1 to 2 minutes without browning. Add the tomatoes, chickpeas, and potato, breaking it up into small pieces with the back of a wooden spoon. Add a little of the cooking liquid from the chickpeas and 1 1/4 cups (300 ml) of water. Simmer for 20 to 30 minutes, until the stew has reduced and thickened a little. Gently fold in the salt cod and parsley, and season liberally with freshly ground black pepper. Taste the stew for salt, although you probably won't need any.