Soak the joint in a bowl of cold water for at least 3 hours. Drain and then place the gammon in a large saucepan. Stud the onion with the 4 cloves and add to the pan with the bay leaves and peppercorns.
Cover the joint with water, bring to the boil, cover and simmer for 2 hours. Meanwhile, zest and juice 1 of the clementines. Place the juice in a saucepan with the zest, stem ginger pieces, sugar syrup from the jar and the mustard seeds. Simmer gently until the mixture becomes a sticky glaze. Set aside.
Preheat the oven to 200°C, fan 180°C, gas 6. Drain the gammon, remove the skin and most of the fat. Place the gammon in a roasting tin. Spoon the glaze over the surface.
Halve the remaining clementines and add to the roasting tin and roast for 45 minutes, basting 3-4 times during the cooking time. Remove the joint from the oven and allow it to rest for a few minutes before carving.