Preheat oven to 200 C / Gas 6. Salt and pepper the duck both inside and out. Stuff the duck with 1 sprig of thyme, 2 cloves of garlic and half of the orange. Rub the outside of the duck with the other sprig of thyme.
Place duck on a rack in a large roasting tin. Roast in the middle of the oven for 1 hour per kilo (so 1 1/2 hours for a 1 1/2kg duck). Baste at least twice with the cooking fat in the tin. Half an hour before the end of the cooking time, place orange slices over the duck and cover with a sheet of foil.
Transfer the duck onto a board to rest and reserve the juices in the roasting tin to make a gravy. Add a little water, salt and pepper and whatever remains of the second sprig of thyme. Boil for 1 minute and pour into a sauce boat or jug for serving.