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 dried chilli flakes, salt and pepper. Bring to the boil, reduce heat to low, and simmer 1 hour.
Add prawns and fish to the stew. Continue cooking 5 minutes, or until fish is easily flaked. Remove from heat, stir in chopped coriander and allow to sit 10 minutes before serving.