Pre-heat the oven to 220°C (425°F, gas mark 7). Wipe the duck dry with kitchen paper. Prick the skin all over with a skewer, or a fork if the tines are sharp, so the fat can run out more easily as it cooks. Season generously with salt and pepper. Sit the duck on a rack over a roasting tin and slide it into the oven. Turn the heat down to 180°C (350°F, gas mark 4) and leave it to cook for 2½hrs.
During that time, all you need to do, whenever you feel like it, is drain off the fat that's gathered in the roasting tin.
Once you've got your first crop of fat, take 1tbsp of it and heat in a frying pan. Add the carrot, onion and celery and sauté until the vegetables are tender and lightly browned. Now add the wine and the herb bundle.
Bring to the boil, stirring thoroughly, then boil hard until reduced by half. Add the stock and boil again until reduced by about a third to a half, giving a syrupy sauce. Stir in the redcurrant jelly until it has melted, then strain into a small pan. Add a little salt and pepper and simmer for about 2 mins. Taste and adjust seasoning. Re-heat when needed.
Peel the turnips, cut into 2cm (¾") cubes and blanch in boiling water for 3 mins. Drain and run under the cold tap. Leave to drain thoroughly.
About 40 mins before the duck is done, drain off most of the fat, leaving a bit in the tin. Add the turnips and turn them in the fat, then leave to roast with the duck.
When the duck and turnips are cooked turn off the oven and leave the door open. Let them rest like this for about 15 mins.
Using a sharp knife or poultry shears, cut the roast duck into four pieces, cutting first from head to tail end, along the breastbone and through the backbone to give two halves, then dividing each half in two.
Serve quickly while still warm with the roast turnips and reheated sauce.
By Sophie Grigson is a celebrity chef and author of Vegetables, published by Collins