This is a relatively straightforward method for cooking a glazed gammon. I am not listing "calculated cooking time" here, I am just going to assume you have a 2kg / 4 pound rolled gammon joint. I might change this at a later date; in the mean time, if your gammon isn't a 2kg one, you should probably look elsewhere.
Preheat the oven to 175 degrees C / 350 degrees F / Gas Mark 4.
Place the gammon in a pan large enough so the gammon is lower than the top, and add enough cold water to cover it. Bring it to the boil, then drain off the water and replace with fresh cold water. Peel the onion and cut it in half, and add it to the pan, with the bay leaves and peppercorns. Bring this to the boil then simmer for 70 minutes (1 hour 10 minutes). Remove the gammon, preserving the stock if desired. Wrapp the gammon in foil and bake in the oven for 40 minutes. Take off the foil, and carefully peel off the rind. Score the fat with a diamond pattern, brush it with mustard and sprinkle demerera sugar over it. This can also be done to the other surfaces if wished. Return to the oven, if possible with the fat on top (but don't worry too much about this) and roast for a further 30 minutes at 225 degrees C / 425 degrees F / Gas Mark 7.
Serve hot or cold. If serving cold, wait for it to cool before slicing to help retain moisture.