Put the gammon in a large pan and cover with cold water. Bring to the boil and skim off any scum that rises to the surface. Reduce the heat, cover, and simmer for 2 hours. Leave to cool in the cooking liquid for 30 mins and then drain.
Meanwhile, make the glaze. Tip all the ingredients into a pan and bring to the boil. Reduce to a simmer and bubble for 10-12 mins or until just syrupy.
Heat the oven to 200C/180C fan/gas 6. Sit the gammon on a board and, using a sharp knife, cut away the skin, leaving behind an even layer of fat. Score the fat all over in a criss-cross pattern.
Transfer the gammon to a roasting tin and roast for 10 mins. Remove from the oven and brush the glaze all over the gammon.
Continue to roast, bastin every 5-10 mins with the glaze, for about 40 mins.
Remove from the oven and leave to rest for 15 mins before carving. Serve with the remaining glaze on the side.