Make the marinade: in a bowl, combine the marinade ingredients; mix until blended; taste for seasoning; adjust with salt and pepper.
Put chicken in a zip-top plastic bag; pour in marinade, turning to coat the chicken.
Make sure the marinade is evenly distributed; seal bag, and refrigerate for 30 minutes to 4 hours.
Preheat oven to 425°; put chicken with the marinade in a large shallow roasting pan.
Place the yams and leek around the chicken, stirring to coat them with the marinade.
Roast for 1 hour to 1 hour 10 minutes, or until it is cooked through with no pinkness remaining and the skin is brown and crispy.
Transfer to a serving platter; cover with foil to keep warm.
To finish the sauce, place the roasting pan over med-high heat; add in the orange zest, juice, and chopped rosemary; reduce the sauce, stirring, for about 2 minutes, until slightly thickened; taste for seasoning.
Pour the sauce over the chicken; spoon the yams and leek around; garnish with rosemary sprigs; serve.