Put the saffron into a small cup, cover with the boiling water and leave to soak.
Heat the oil in a flameproof casserole, add the onion and peppers and fry gently for 5 minutes until just beginning to brown. Stir in the tomatoes, fish stock and wine. Mix in the tomato paste and some seasoning, then add the potatoes and saffron with its soaking water. Bring to the boil then cover and simmer for 20 minutes until the potatoes are just tender. Allow to cool, cover and chill until required.
To make the gremolata, mix together the parsley, lemon rind and garlic, cover and chill until required.
When ready to serve, reheat the sauce then add the cod and seafood. Cover and simmer gently for 15 minutes until the cod is just tender. Be careful not to overcook or the cod will fall apart. Sprinkle with a little of the gremolata, then ladle into shallow bowls. Serve immediately with warm rustic bread, salad and the remaining gremolata for sprinkling.