Peel the prawns and reserve the shells. Mix shells with the spice mixture and water and bring to the boil in a large stock pot. Reduce the heat and allow to simmer for about 20 minutes.

Melt the butter or margarine and, when foaming, add the onion, green pepper, garlic and flour. Cook slowly, stirring constantly until the flour is a pale golden brown. Gradually strain on the stock, discarding the shells and spice mixture. Add the thyme and bay leaf and stir well. Bring to the boil and then simmer until thick.

Add the parsley and the Worcester sauce to taste. Add the oysters, peeled prawns and tomatoes and heat through gently to cook the oysters.

Adjust the seasoning and serve over rice.

If the mixture is not thick enough, use equal portions of butter or margarine and flour mixed together to a paste. Add a bit of the paste at a time to the mix, and boil in between additions until the desired thickness is reached.