Soak the clams in a bucket or bowl 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. Drain the clams and put in a large heavy-based saucepan with 250ml (9fl oz/1cup) water. Cover and simmer over low heat for 5 minutes, or until the clams open. Discard any that do not open. Strain and reserve the liquid. Remove the clam meat from the shells.
Heat the oil in a clean saucepan. Add the bacon, onion and garlic and cook, stirring, over medium heat until the onion is soft and the bacon is golden. Add the potato and stir well.
Measure the reserved clam liquid and add water to make 310ml (10¾fl oz/1¼cups). Add to the pan along with the stock and milk. Bring to the boil, then reduce the heat, cover and simmer for 20 minutes, or until the potato is tender. Uncover and simmer for 10 minutes, or until slightly thickened. Add the cream, clam meat and parsley and season to taste. Heat through gently before serving, but do not allow to boil or the liquid may curdle.