1) Preheat oven to 220°C / gas mark 8.
2) Whisk together pomegranate treacle, horseradish, mustard, salt, and coarsely ground black pepper. Set aside and let sit at room temperature for at least 30 mins before using to allow the flavours to meld.
3) Remove the neck, heart and gizzard from inside of the turkey. Rinse the bird inside and out with cold water and pat dry. Rub the entire surface of the bird with the butter and season well (including the cavity) with salt and pepper. Truss the turkey and place breast-up on a rack in a large roasting pan. Roast for about 45 mins, until slightly golden brown. Reduce the heat to 180°C/ gas mark 4 and continue roasting, basting with some of the chicken stock every 15 minutes, for about one hour and fifteen minutes longer. (An instant-read thermometer inserted in the thigh should register 71°C.)
4) During the last 15 minutes of roasting, stop basting with the chicken stock and brush the entire turkey with 240ml of the pomegranate glaze. Remove the turkey from the oven, brush with the remaining glaze, loosely tent and let rest 15 minutes before slicing.