Place the chicken in a large heavy-based stockpot. Wrap the peppercorns, bay leaf, parsley stalks and thyme in a small square of muslin and tie with kitchen string to form a pouch. Add to the pan with the water.
Cover and bring slowly to the boil over medium heat. Reduce the heat slightly and simmer for 1 hour.
Meanwhile, heat the oil in a large frying pan over medium-low heat. Cook the leek, carrot and celery for 10 mins, stirring often, until tender.
Transfer the vegetables to the stockpot. Simmer for a further 1 hour.
Remove from heat. Carefully transfer the chicken to a board (in pieces if necessary). Cool slightly, then remove and discard skin and bones. Coarsely chop the meat.
Return the chicken to the soup. Add the pasta and cook for 10 mins or until the pasta is tender. Season. Sprinkle with parsley leaves.