Preheat the oven to 220C/425F/Gas 7.
Rub the butter all over the bird and season well with salt and freshly ground black pepper. Then lay the bacon rashers over the bird, more or less covering it completely. The job of the bacon is twofold: to help keep the breast basted without you having to open (and therefore cool) the oven every ten minutes; and to buy you time, so the skin doesn't burn before the breast is cooked through.
Place the bird in a roasting tray in the centre of the oven. Cook on this high heat for 25-30 minutes until the bacon is almost burnt.
Remove the bacon from the breast of the bird and put on one side. Baste the breast of the bird with the fat and juices in the pan and return to the oven. Turn down the oven to 180C/350F/Gas 4 and continue to cook for about 12 minutes per 1kg/2¼lb, protecting the skin with buttered foil only if it seems to need it. Cook until the juices run clear when the thickest part of the breast is pierced with a skewer and the skin is brown and crispy. Cooked like this even a big bird (say 6-7kg/13½-15¾lb legless weight) shouldn't take more than 1½-2 hours in total.
Serve slices of roast turkey breast and spoonfuls of the leg meat in their rich gravy, with all the usual trimmings: roast potatoes, bread sauce, steamed sprouts, chipolatas wrapped in bacon and stuffing.