Place gammon with the net in a pot and add water, bay leaves and peppercorns. Make sure that the gammon is submerged completely in the water. Bring to the boil, then reduce to a simmer for 90 minutes. Drain and cool.
Pat dry, remove the net and pull off the skin to expose the fat. Score fat about ½ cm deep, taking care not to cut through to the meat. Place in a roasting pan.
Preheat oven to 200°C. Make the glaze by melting the butter in a saucepan over medium heat, add the rest of the ingredients and whisk together. Baste the gammon with the glaze and roast for 20 minutes.