Preheat the oven to 180°C / 350°F / Gas Mark 4, 10 minutes before cooking. Wipe the turkey joint and weigh, allowing 20 minutes per kg plus 70 minutes. If necessary, tie a neat shape with some fine twine then place in a deep dish.
Blend together the garlic, ground cinnamon, ginger and 1 tablespoon of the demerara sugar. Blend to a thick paste with the soy sauce. Brush over the turkey joint and leave in the fridge to marinate for at least 30 minutes.
When ready to cook, place the joint in a roasting tin and pour round the stock. Cook for 1 hour 25 mins, occasionally basting with the stock. (Cover lightly with tin foil if top is becoming too dark)
Remove from the oven and press the remaining sugar on top of the turkey joint. Return to the oven and continue to cook for a further 10 minutes or until thoroughly cooked. Discard twine if used and serve with the freshly cooked vegetables.