Obtain from the fishmonger fish trimmings such as heads and backbones. Rinse well and place in a large pot. Add water, salt and half teaspoon peppercorns and simmer for 2 hours. Strain stock and reserve. Gently fry onion, garlic, chopped leeks, carrots and celery in oil for 10 minutes. Add chopped tomatoes, herbs tied into a bunch and reserved fish stock. Season well with salt and freshly ground pepper. Cover and simmer for 30 minutes. Add potatoes and boil for 5 minutes. Reduce heat, add fish pieces and simmer 3 minutes, then add lobster pieces and prawns and simmer for 7-10 minutes until prawns turn pink and lobster meat is cooked. Do not boil when shellfish are added for they will toughen. Pile fish, shellfish and potatoes in a serving dish, sprinkle with chopped parsley and keep hot. Serve soup in a tureen as a first course with crusty bread, then follow with seafoods and potatoes served with lemon wedges and a cruet of olive oil. Alternatively soup, potatoes and seafoods may be served together in deep plates. NOTE: Fish and shellfish preparation: Fish suitable are snapper, mullet, eel, whiting, john dory and bream. Cut into 5 cm (2 inch) slices and sprinkle with lemon juice. Add heads and trimmings to fish stock. Cut rinsed lobster tails into 5 cm (2 inch) pieces with shell on. Shell and de-vein prawns.