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The evening before you want to serve, put the gammon in a large container and cover with water to soak overnight. Drain and cover with fresh water at least twice.
The next day, put the gammon in a large saucepan and pour over the ginger beer, topping up with water until fully submerged. Add the ginger, spring onions, garlic halves, bay leaves, pimento berries, black peppercorns and tamarind paste, and bring to the boil. Reduce the heat to a simmer, cover and cook for 1 hr 30 mins. Turn the heat off and leave to cool in the pan, then transfer to a shallow baking tray lined with baking parchment. Reserve the cooking liquid.
Meanwhile, strain 800ml of the reserved cooking liquid into a saucepan to make the glaze. Cook over a high heat for 20 mins, or until reduced to about a quarter of its original volume. Add the tamarind paste, sugar or honey, spices, mustard and vinegar, and continue to cook until thick and glossy. Leave to cool slightly. Heat the oven to 180C/160C fan/gas 4.
Cut the string off the ham and remove the skin. Trim any excess fat, leaving a ½cm-thick layer on the ham. Score it in a diamond pattern using a sharp knife. Brush over the glaze, then bake for 10 mins. Baste again and repeat until the ham is dark golden and all the glaze has been used, about 20 mins. Leave to cool before slicing and serving.