Pierce skin of duck all over with a fork to allow fat to drain and help skin crisp.
Mix together garlic, chillies, sage, thyme and rosemary.
Rub duck pieces with half the herb mixture.
Arrange duck pieces in a shallow baking pan, large enough to hold them in one layer.
Drizzle duck pieces with 125 ml oil, seasoned with salt and pepper.
Turn duck pieces over, rub with remaining herb mixture and drizzle with another 125 ml of seasoned oil.
Marinate duck pieces, covered with plastic wrap and chilled, for eight to 24 hours, turning pieces once.
Bring duck to room temperature before cooking.
Place duck pieces in a large grilling pan with a draining rack. (If desired, dot pan with butter.)
Grill duck pieces, skin side down, uncovered, under a hot grill for 10 minutes.
Turn pieces and grill for another 10 minutes (grill duck pieces in two batches if necessary).
Transfer pieces, skin side up, to a shallow baking pan.
Cover and roast in the middle of the oven at 180 °C for 20 to 30 minutes for medium-rare or, continue roasting to desired doneness.
After removing from oven, allow duck pieces to stand for 10 minutes.
Serve with mashed potatoes, garnished with herbs.