Make stuffing: heat oil in a pan and fry onion for 4 minutes. Add cranberries, lemon zest and juice; remove from heat. In a large bowl, mix remaining stuffing ingredients with onions; season.
Prepare trimmings: stretch bacon rashers with back of a knife; cut in half. Wrap a piece of bacon around each of the sausages and prunes. Place seam side down in a roasting tray, cover and chill until required.
Stuff the turkey: Preheat oven to 180°C/Fan 170°C/350°F/Gas Mark 4. Spoon stuffing into neck cavity of turkey, then tuck skin flap underneath; secure with cocktail sticks. Roll any remaining stuffing into balls and roast with bacon rolls.
Truss and weigh turkey: tie string around leg joints, then around parsons nose, under the bird and up over the wings. Tie tightly to secure shape. Weigh turkey, then calculate cooking time: allow 20 minutes per kg + 90 minutes. Place in a large roasting tin. Spread all over with butter and season. Loosely cover with foil.
Roast turkey for the calculated time, basting occasionally with the pan juices. Remove foil for final 45 minutes, to crisp skin; roast bacon rolls and stuffing balls above the turkey for 20 minutes. Check turkey is cooked - pierce thigh with a skewer – juices should run clear. Transfer turkey to serving plate, cover with foil and rest in a warm place for 20 minutes – this makes carving easier.
Make gravy: spoon off excess fat from juices in roasting tin. Place tin on a low heat; add flour and cook, stirring for 1 minute. Gradually stir in stock and bring to boil. Simmer for 5 minutes, stirring occasionally; strain into a warm gravy boat.