Pour the stock into a large saucepan and add the carrots, potato, leeks and orange zest. Bring to the boil over a high heat, skimming the surface as necessary, then reduce the heat to medium and leave the soup to bubble for about 20 minutes or until all the vegetables are very tender.
Remove and discard the strips of orange zest. Cool slightly, then purée the soup in a blender or food processor until smooth.
If serving the soup hot, return it to the rinsed-out saucepan. Reheat and add the orange juice, then adjust the seasoning. Ladle the soup into bowls and add a spoonful of yogurt to each. Sprinkle with the parsley and shredded orange zest and serve at once.
To serve the soup chilled, leave to cool, then chill for at least 4 hours. When ready to serve, stir in the orange juice, then adjust the seasoning. Garnish and serve as for the hot soup.