Put the gammon joint into a large saucepan, cover with cold water and bring to the boil over high heat. Drain the water from the saucepan, add fresh cold water, the onions, carrots, celery, bay and peppercorns and bring to the boil once more. Reduce the heat, cover with a lid and simmer the gammon and vegetables gently for 20 minutes per 500g/1lb 2oz. (If your pan is not quite large enough to cover the joint completely, turn the gammon over halfway through the cooking time.)
Take the gammon off the water and set aside to cool for 15 minutes. (The cooking liquid can be strained and reserved for making soup for another time.)
Preheat the oven to 200C/180C Fan/Gas 6 and line a large roasting tin with kitchen foil.
Use a small knife to remove the rind from the gammon joint, leaving as much of the fat intact as possible. Score the fat in a diamond pattern, and push a clove into the center of each 'X'.
Place the gammon into the foil-lined roasting tray, ensuring that the sides of the foil come halfway up the joint to contain any roasting juices.
For the glaze, mix the honey and mustard together and brush half of it evenly over the gammon, including one side of the joint. Roast in the center of the oven for 10 minutes, then take the joint out and brush the top and remaining side with the rest of the glaze. Return the gammon to the oven for a further 10– 15 minutes, turning the tin so that the opposite side of the gammon faces the back of the oven.
The gammon is ready when the fat on top is glossy and golden brown. Cover loosely with foil if the top begins to look too brown. Remove the gammon from the oven and set aside to rest for 15 minutes before carving.
Pour any juices that have collected in the kitchen foil into a small pan, and warm through gently.
Carve the gammon, serve on a large platter and spoon over the warmed juices.