Preheat oven to 200C/390F.In a mixing bowl marinate the chicken in the salt, pepper, garlic powder and olive oil.Place the chicken on a lined cookie sheet and roast for 20-30 minutes, depending on the size of the thighs.Remove from the oven and set aside to cool for 20 minutes.Cut the cooled chicken into a small dice, around 1/2in/1cm cubes.Place the chicken in a mixing bowl with the diced celery, parsley and mayonnaise.