Trim leek, cut in halves lengthwise, wash thoroughly, slice finely.
Melt butter in saucepan, add leek, cook few minutes, stirring constantly, add crumbled stock cube and water, cover, simmer 10 minutes.
Place well-drained potatoes and leek mixture in blender or food processor, blend until as smooth as desired.
Return mixture to saucepan with undiluted soup, milk and cream, bring slowly to boil, season with salt and pepper.
Add parsley and oysters just before serving, reheat gently.