Open oysters and pour off all juices into a small bowl. Remove oysters from shells and set aside. Strain oyster juices through damp, fine muslin and set aside.
Wash leeks carefully, and cut into 1 cm (½ in) lengths. Melt butter in a large pan, add leeks and cook gently until they soften. Peel and slice potatoes, and add to leeks with chicken broth. Bring to the boil, then reduce heat to