Thaw duck in refrigerator until partially dethawed. The skin should be soft, but the insides should still be frozen. Take duck out of wrapping and stab the duck 15 to 20 times. Make sure the stab holes go past the fat and into the meat. This is very important for the marinating process.
In a large mixing bowl, mix 250ml sherry and 3 tablespoons olive oil. Place duck in the mixing bowl. Spread the sherry and olive oil all over the duck. Cover the mixing bowl and refrigerate until duck is completely thawed (works best overnight).
Preheat oven to 190 C / Gas 5. Remove duck from marinade and remove the neck bone and the giblets from inside the duck. Spread some of the marinades inside of the duck. Rub the outside of the duck with 3 tablespoons olive oil and oregano, rosemary and basil. Pour remaining sherry into bottom of roasting tin and place duck in tin.
Roast for about 30 minutes per 225g (cooking times may vary). Baste every 30 to 45 minutes.
If you choose to stuff the bird with dressing, do this after spreading marinade inside the duck.