Preheat the slow cooker if necessary; see the manufacturer's instructions. Put the gammon joint into the slow cooker pot. Tuck the vegetables around the sides, then add the bay leaves, cloves and peppercorns. Pour over the boiling water to just cover the gammon.
Cover with the lid and cook on high for 5–7 hours or until the gammon and vegetables are cooked through and tender. Lift the gammon out of the slow cooker pot and transfer to the base of a grill pan. Cut away the rind and discard.
Mix the honey and mustard together for the glaze, spoon over the top and sides of the joint, then add 3 ladlefuls of stock from the slow cooker pot to the base of the grill pan. Grill until the gammon is golden brown. Carve into slices and serve with the sauce from the grill pan and drained vegetables from the slow cooker pot. Accompany with steamed green beans, if liked.