Trim off the leafy tops of the celery and reserve. Cut the celery into 5mm slices. Tie the parsley, thyme and bay leaf together with kitchen string to make the bouquet garni.
Pour the 4 cups water into a wide saucepan. Add peppercorns, celery tops, onion, carrot and bouquet garni. Bring to the boil and simmer 10 minutes. Add the chicken and simmer 15 minutes. Cool in the poaching liquid, then tear into chunks.
Heat oven to 180 degrees C. Bake the chopped walnuts until crisp, 5-8 minutes.
Cut the apples into small cubes and add to a salad bowl. Add the lemon juice and toss well to prevent the apples from browning.
Add the chicken pieces, celery, mayonnaise, yoghurt and 2/3 of the walnuts. Stir well and adjust seasoning to taste. Cover and chill one hour. Garnish with the remaining walnuts before serving.