Sit the gammon with onion, bay leaves and vinegar. Pour enough water to completely cover the gammon.
Bring to the boil, then simmer for 45 minutes. Remove gammon from the pot and allow it to cool.
Once cool enough to handle, carefully cut away skin, leaving layer of fat. Pat skin dry with a tea towel, then transfer to a wire rack and leave to dry out.
Heat oven to 180C/160C Fan/ Gas Mark 4. Line one small roasting tin with foil and two small baking tray with baking parchment.
Put gammon in foil-lined tin, fat-side and rub all over with mustard then sugar.
Put the skin onto one of the baking trays, then place the other piece of parchment paper followed by the second tray. Find a weight to go on the top such as a heavy pan then place gammon and skin in the oven.
Roasted for 30 minutes, basting twice, until gammon is golden.
Take out of the oven and turn the oven up to 220C/200C Fan/ Gas Mark 7. Continue cooking the skin turning into crackling after 10 minutes it will be deep golden and crisp.