Put the clams in a large heavy-based saucepan with 250ml (9fl oz/1cup) water. Place over high heat, bring to the boil, then reduce the heat to medium, cover and simmer for 5 minutes, or until the clams open. Discard any that do not open. Strain and reserve the liquid. Add enough fish stock to make 1litre (35fl oz/4cups).
Rinse the saucepan, then return to the heat, add the butter and cook the onion, celery, carrot and leek over medium heat, covered, for 10 minutes, stirring occasionally. Add the stock and bay leaf, bring to the boil, then reduce the heat and simmer for 10 minutes. Add the rice, return to the boil, then cover and cook for 15–20 minutes, or until the rice and vegetables are tender.
Remove all but eight of the cooked clams from the shells. When ready, remove the soup from the heat and stir in the clam meat. Remove the bay leaf. Cool slightly, then process in batches in a blender until smooth (a food processor can be used but it will not achieve the same smooth consistency). Rinse the saucepan. Pass the soup through a sieve back into the pan. Stir in the cream and season with salt and pepper. Gently reheat and serve, garnishing with the reserved clams.