Scrub the mussels and clams in cold water. Scrape off any barnacles from the mussels and pull away the dark hairy beards. Discard any mussels or clams with damaged shells or open ones that do not close when tapped firmly with a knife. Set aside.
Heat the oil in a large saucepan and gently fry the onions and garlic for 5 minutes. Stir in the chilli flakes, then add the mussels and clams with the squid and king prawns and stir well.
Stir in the hot stock, wine, saffron, tomatoes, bay leaf and sugar and season with salt and pepper. Cover and cook for 5 minutes. Discard any mussels or clams that remain closed.
Add the fish, re-cover and cook gently for 5 minutes until cooked through. Serve immediately.