Soak the clams in a bucket of salted cold water for 1–2 hours to rid them of sand, then rinse under running water. Discard any clams that are broken or any that are open and do not close when tapped on the work surface.
Put 1cm (½inch) water in a large saucepan and bring to the boil. Add the clams, cover and cook for 3 minutes, until they all open — discard any closed ones. Take out the clam meat and strain the liquid into a bowl.
Heat the oil in a frying pan over low-medium heat, add the bacon, onion, celery, potato and leek and fry for 10 minutes. Add the bay leaf, the reserved clam liquid, tomatoes and stock. Simmer for 10 minutes, or until the potato is cooked. Season, then stir in the clams. Remove the bay leaf and serve.