Place the prawn shells in a large pot and cover with 2 litres of water. Cover and simmer over medium low heat until liquid is reduced by half. Strain out the shells.
Select a large stock pot capable of holding all the ingredients. Add oil to the pot and heat over medium high heat. Using a long handled spoon, stir in flour; cook and stir for several minutes until dark brown. At that point, the flour suddenly puffs and absorbs the oil. Slowly stir in chicken stock and an equal amount of water. Place chicken in the pot. Add onions, celery, pepper, tomatoes, garlic, bay leaves, cayenne pepper, savoury seasoning and salt. Boil for about 1 1/2 hours or until chicken meat is no longer pink and the juices run clear.
Remove chicken from the pot and set aside until cool enough to handle. Remove bones and chop the chicken into about 2.5cm pieces.
Add chicken, prawns, stock, crab meat, sausage and prawn the pot. Add the file powder and stir from the bottom of the pot. When the gumbo comes to a boil, remove from heat. Continue to stir from the bottom for 1 minute. Serve.
File powder is also known as sassafras leaves. It can be purchased in Hispanic speciality stores or online. Alternatively, you can thicken the sauce with a mixture of cornflour and water.