Heat 2 tablespoons of olive oil in a large sauce pan over a medium heat. Add the tomatoes, onion, celery and garlic and cook 1-2 minutes. Then stir in the tomato purée, wine, water, the bouquet garni and saffron. Bring to a simmer and allow to cook for 10 minutes. Add the cubed potatoes and cook for another 10 minutes.
Then add the whiting, mussels and prawns. Cover the pan and simmer very gently for 10 minutes. Taste and season with a little salt and black pepper.
Meanwhile make the rouille: Put all the ingredients except the oil in a food processor and whiz. Then, slowly pour in the oil until you have the consistency of mayonnaise. Taste and season.