Preheat the oven to 200°C, fan 180°C, gas 6. In a bowl, mix together the butter, thyme, sage and garlic, and season with freshly ground black pepper.
Use your fingertips to separate the turkey skin from the breast meat, being careful not to tear it. Push in the herb butter and spread evenly over the breast. Lay the bacon over the turkey, overlapping as you do so.
Tie the turkey legs together with string and place the bird on a rack inside a large roasting tin. Roast for 30 minutes, uncovered, then remove from the oven and loosely cover with buttered foil. Reduce the temperature to 180°C, fan 160°C, gas 4. Continue roasting for 3-3½ hours, basting every 45 minutes with the juices, until cooked through. Remove the foil a few minutes before the end of cooking if the skin still needs to crisp up.
Mix all the stuffing ingredients together and season. Shape into 20 balls and place in the oven in a roasting tin, 25 minutes before the turkey has finished cooking.
To test whether the turkey is cooked, insert a skewer into the thickest part of the leg – the juices should run clear. Transfer to a warmed serving plate, cover loosely with foil and leave to rest for 30 minutes before carving. Garnish with the sage leaves and serve with the stuffing balls.
Cook’s tip: to baste a turkey: use a bulb turkey baster, a basting brush or a large spoon to scoop up the juices and drizzle over the turkey.