Place the gammon joint into a large lidded saucepan, cover with cold water and bring to the boil over a high heat. Drain the water from the saucepan, add fresh cold water, add the onions, carrots, celery, bay leaves 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.)