Place the whelks in a pan of well-salted water, bring to the boil and simmer for 4 minutes. Remove the whelks and set aside. Bring the winkles to the boil and drain at once.
Place the mussels in a large saucepan and cover with a lid. Place the pan on a fierce heat, and as soon as the mussels open, remove from the heat, strain off the juice and reserve. Repeat with the clams and oysters.