1/4 cup finely chopped fresh flat-leaf parsley leaves
Place 3 1/2 cups cold water, stock, chicken, onion, celery and leek in a large saucepan over medium-high heat. Bring to the boil. Reduce heat to medium-low. Simmer, covered, for 1 hour.
Transfer chicken to a bowl. Cool slightly. Remove and discard bones. Shred meat. Set aside. Strain stock mixture into a large heatproof jug. Discard solids.
Melt butter in pan over medium-high heat. Add flour. Cook, stirring, for 1 minute or until bubbling. Gradually stir in stock mixture and cream. Cover and bring to the boil. Reduce heat to medium. Add chicken. Cook, stirring occasionally, for 15 minutes or until soup thickens slightly.
Season with salt and pepper. Sprinkle with parsley. Serve.