Heat the oil in a large heavy-based saucepan. Add the onion and leek then cook over a moderate heat, stirring frequently, for 5 minutes, until beginning to soften. Stir in the garlic and celery and cook for 2 more minutes.
Add the tomatoes, tomato paste, fish stock, wine and bay leaf. Bring to the boil, then reduce the heat. Cover and simmer for 30 minutes. Season with salt and pepper.
Stir in the whiting and ling, cover and cook for 5 minutes, then stir in the seafood. Re-cover and cook gently for a further 2–3 minutes until the white fish is cooked and the seafood is heated through.
Remove the bay leaf and, if you are using it, stir in the liqueur. Ladle the stew into a warmed tureen or individual bowls and sprinkle with chopped basil to serve.