Make the sauce first. Peel the grapefruit with a very sharp stainless steel knife, working over a bowl to catch all the juice. Remove all pith and cut the segments from their skin. Squeeze the juice from all the pith and skin, which should amount to 300 ml/½ pint. Place the sugar in a dry saucepan over a gentle heat. When it begins to melt, stir until it is caramelised. It will start to bubble and rise in the pan. Remove from the heat and continue stirring until it subsides. Pour in the grapefruit juice at arm’s length as it may splatter alarmingly Return to the heat and add the honey, soya sauce, lemon juice and grated ginger. Stir until simmering and the caramel has dissolved. Blend the cornflour and the water to form a smooth paste and gradually stir into the sauce until it becomes thick and clear. Add a good grinding of black pepper and salt to taste. Finally, add the grapefruit segments and remove from the heat.
Prick the duck’s skin all over. Place the breasts skin side up on a rack in a roasting tin, with a little water in the bottom. Mix the honey and the soya sauce together and brush over the skins. Cook in a preheated oven at 200°C/400°F/Gas 6 for 30 minutes until the skin is crisp.
Remove from the rack and carve the breasts diagonally into 5 mm/¼ in slices. Arrange on a fine warmed platter and pour the heated sauce over the duck. Serve with nice little potatoes, baby carrots and peas.