Heat the oil in a large pot over medium heat. Mix in the celery, onion, carrots and garlic. Cook and stir until onion is tender. Stir in the tomatoes, water, wine, fish stock and sherry. Dissolve stock cubes in the stew and season with chilli flakes, salt and pepper. Bring to a boil, reduce heat to low and simmer 1 hour.
Mix the coriander, prawns and fish into the stew. Continue cooking 5 minutes, or until prawns are opaque and fish is easily flaked. Remove from heat and allow to sit 10 minutes before serving.