Place the duck in a roasting tin, and prick the fleshy parts with a skewer.
Season all over with salt and pepper, then roast it on the highest shelf of the oven for 20 minutes.
After that reduce the heat to gas mark 4, 350°F (180°C), and continue to roast for a further 2½ hours. From time to time during the cooking remove the tin from the oven and drain off the fat into a bowl (it's wonderful for roasting potatoes).
Ten minutes before the end of the cooking, mix the marmalade and port to make the glaze and brush all over the skin of the duck and return it to the oven for its final cooking. Remove the duck to a carving board to rest while you make the sauce.
Strain off all the excess fat from the roasting tin, then place the tin on a gentle heat and add the coriander seeds and garlic. Let these cook together for a minute or two as you stir and scrape off all the residue from the sides of the tin. Now pour in the stock, lemon juice and orange juice and zest, bring to simmering point, taste and season and simmer gently for 15 minutes.
During the last 5 minutes carve the duck by using a sharp knife to cut it into quarters. Arrange these on a warmed serving plate. Finally, stir the marmalade and the port into the sauce and pour it on to the duck.
Garnish with watercress and serve immediately.
Pre-heat the oven to gas mark 7, 425°F (220°C).
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