Place chicken, onion, peppercorns and thyme in a large pot and cover with 3 litres water. Bring to the boil, reduce heat and simmer for 30 minutes until chicken is cooked through.
Strain chicken and seasonings through a colander, reserving the liquid. Remove meat from chicken bones. Shred and set aside.
Heat the oil in a large pot and add leek, carrots, celery and garlic. Cook until just tender. Add the Soup Mix and potatoes with 2 litres of the reserved cooking liquid. Simmer for approximately 30 minutes until soup mix is tender. Return chicken meat to pot and cook for a further 5 minutes until heated through. Season to taste and finish with parsley.