Season chicken with salt & pepper. Place in a marinating dish or plastic bag.
Mix together the oil,mustard, vinegar, thyme, rosemary & brown sugar.
Pour mixture over chicken, mix gently until chicken is coated. Add lemon halves, and marinate in fridge for 1-2 hours, covered or plastic bag well closed.
Preheat oven to 400 degrees.
Drain chicken, saving the marinade. Leave chicken in fridge while you cook the potatoes.
Toss garlic cloves & potatoes in marinade, then transfer to a roasting pan and roast, uncovered, for 20 minutes.
Place chicken skin-side up in the roasting pan, add lemons, and return to oven. Roast for 40-45 minutes, basting with the marinade occasionally, until chicken and potatoes are cooked through and crisp.