Place the gammon in a large saucepan of cold water and leave to soak for 2–12 hours depending on the amount of salt in the cure (soak overnight if in doubt).
Drain the gammon, then weigh it and calculate the cooking time based on 25 minutes per 500 g (1 lb), plus 20 minutes. For a joint over 3 kg (6½ lb), allow 20 minutes per 500 g (1 lb), plus 20 minutes. Return it to the pan and cover with fresh cold water. Add the bay leaves and 2 tablespoons of the sugar and bring to the boil. Cover, reduce the heat and simmer for half the calculated cooking time.
Remove the gammon from the water and strip off the skin. Stand the joint on a large sheet of foil in a roasting tin and score the fat diagonally in a trellis pattern. Mix the marmalade and remaining sugar and spread over the fat. Pour the ginger ale around the joint and enclose in the foil, sealing the edges firmly.
Bake in a preheated oven, 190°C (375°F), Gas Mark 5, for the remaining cooking time. During cooking, baste the gammon with the ginger ale, then rewrap in the foil. About 20 minutes before the end of the cooking time, fold back the foil, baste again and return to the oven. Remove and leave to cool overnight.