Preheat the grill to a low heat. Put the chicken on an oiled baking tray. Spoon over half the dressing and grill for 6 minutes. Turn the chicken over, spoon over the remaining dressing and grill for 6 minutes, or until cooked. Warm the pittas under the grill so they puff up. As soon as they are cool enough to handle, split them open and fill with the lettuce, chicken fillets and yoghurt dressing before serving.