Melt butter in a heavy pot over medium heat. Add the next 6 ingredients and cook, stirring frequently, until softened, about 5 minutes. Add diced tomatoes, clam nectar, water, tomato paste and sugar and bring to a boil. Reduce heat and simmer 20 minutes. Stir in the fish, bay leaf, allspice, thyme, hot pepper sauce and salt and pepper to taste. Simmer, stir occasionally to break up the fish, 20 minutes. Stir in cornstarch mixture and simmer until thickened, about 2 minutes. Stir in Worcestershire sauce, rum and sherry. Remove from heat and let stand, covered, 1 hour to develop flavour. Gently return to a simmer, and then spoon into bowls and serve.