Make a court bouillon from the fish stock, bay leaf, white wine and mirepoix: place together in a saucepan, bring to the boil and simmer for 5 minutes. Remove the intestinal tract from the crayfish by pinching the middle section of the tail between your thumb and forefinger, then twist and pull. Plunge the crayfish into the court bouillon and cook for 4 minutes. Remove and allow to cool, then strain, reserving the liquid.
Shell the crayfish tails and save all the shells and heads; crush the heads. Put the shelled meat in a little of the cooking liquor; either use the meat for garnishing the dish for which you are making the sauce or, if it is only the sauce you want, use the meat for another dish. Heat the oil in a heavy-based saucepan, add the shallot and sweat for about a minute, stirring to prevent browning. Add the crayfish heads and shells, tomato and garlic, and continue to sweat for a further minute. Pour in the brandy, ignite and allow to burn out, then stir in the tomato purée and pour in the court bouillon. Bring to the boil and simmer for 20 minutes.
After 20 minutes, place the sauce in a food processor or blender and blend for a few seconds. Strain the sauce through a sieve into a saucepan. Return the sauce to the boil and reduce until it thickens and only about 450 ml/¾ pint remains.
To finish the sauce, add an equal quantity of double cream, return to the boil and continue boiling until it thickens. Strain through a sieve before serving.