Melt 250g butter in a heavy saucepan. Cook the celery and onion until tender. Remove from the heat and toss mixture with the remaining stuffing ingredients.
To roast the turkey:
Preheat oven to 135 degrees C.
Stuff the turkey just before roasting - don't do it ahead of time. To stuff it, fasten the neck skin to the back with a skewer or poultry pins. Fold the wings under the back. Fill the body cavity lightly with the stuffing - don't over stuff as the stuffing will expand while cooking. Tie the drumsticks together at the tip, under the tail.
Place turkey, breast side up, on a rack in a roasting tin. Brush with melted butter.
Roast, uncovered, for 2 1/2 - 3 hours. If you are using a meat thermometer, it's done when the thermometer reads 82 degrees C in the thickest part of the thigh. If not, check for doneness from 2 1/2 hours on by cutting into the centre of a thigh and seeing if the juices run clear. As you are cooking, cover the turkey with a tent of aluminium foil when it just begins to turn golden.
When done, remove from oven and let stand 20 minutes before carving.