Allow the turkey to come to room temperature before cooking. Season it well inside and out with salt and black pepper.
Pre-heat the oven to 180C/160C fan/gas mark 4.
Blend the butter with the clementine zest, the 2 tablespoons of chopped sage and the 2 cloves of crushed garlic. Add salt and pepper to taste and blend well again.
Using your fingertips, carefully lift the turkey skin away from the meat taking care not to tear it. Push all but 2 tablespoons of the flavoured butter under the skin, pushing it back as far as possible under the skin towards the neck cavity. Smooth the skin to create an even layer of herby butter under the skin. Brush the remaining butter all over the outside of the turkey.
Arrange the bacon over the breast of the turkey in a criss-cross formation. Carefully push several sage leaves under the bacon and the turkey, so half the leaf is exposed and half is under the bacon.
Place two clementine halves in the cavity with the remaining sage, bashed garlic cloves and a few rosemary sprigs if using.
Truss the turkey by wrapping string around leg joints, then around the parsons nose, under the bird and up over the wings. Tie securely where the legs cross over.
Weigh the turkey and calculate the cooking time at 20 minutes per kg plus 90 minutes. For a 4.5kg turkey this will be 3 hours.
Put the turkey in a roasting tin along with the remaining clementines and whole garlic bulbs. Cover the tray tightly with tin-foil making sure there’s a slight gap between the turkey and the foil. Place in the pre-heated oven.
After 90 minutes, uncover the turkey and baste it in its juices before re-covering and returning to the oven. If you have a very large turkey you may want to baste a second time during the cooking.
Remove foil for the final 45 minutes of cooking to crisp skin. To check the turkey is cooked either use a meat thermometer, or, pierce at the thickest part with a skewer. The juices will run clear when cooked.
Transfer the turkey to serving plate. Cover with a new layer of foil and a warm damp tea towel. Leave it to rest in a warm place for at least 30 minutes before carving.