1. Preheat broiler, heat a charcoal grill until coals form white ash, or preheat a gas grill to medium.
2. To prepare sauce, in a small saucepan, combine ketchup, vinegar, horseradish, brown sugar, garlic and thyme. Mix well. Bring to a boil over medium-low heat. Cook, stirring frequently, until thickened, about 5 minutes. Remove from heat; stir in pepper.
3. Brush tops of chicken pieces lightly with sauce. Place chicken, sauce-side down, on a foil-lined broiler pan or grill rack. Brush other sides lightly with sauce.
4. Broil or grill 8 cm from heat, basting with remaining sauce and turning until no longer pink in center, about 5 to 7 minutes per side. Let chicken stand for 5 minutes before serving.