Bring a large pot of lightly salted water to a boil; add stock cube and stir until melted. Add chicken and poach for about 13 to 14 minutes. Remove chicken and leave liquid in pot; cool chicken and cut into 1cm chunks; reserve.
Add mushrooms to liquid and cook over medium heat; cook for a few minutes and remove mushrooms from stock; reserve.
With remaining stock in pot, add some more water if needed. Bring water to a boil and add pasta to cook for 8 to 10 minutes or until al dente; drain and discard stock. Cool pasta with running water; reserve.
In a small bowl, combine sour cream, mayonnaise, garlic, curry powder and salt and pepper to taste.
In a large bowl, combine cooled pasta, chicken, mushrooms, sour cream dressing, olives, apple, yellow capsicum and celery; mix well. Refrigerate for at least 3 hours and serve.
Fresh garlic can be used instead of garlic powder.