Put the onion, carrot and celery into the slow cooker and then rest the gammon on top. Pour in the cola until the gammon is totally submerged. (I have cooked large gammons before where the cola hasn't quite covered it, and it's fine). Drop in the bay leaf, cinnamon stick and a good crack of black pepper. Cook on low for 8 hours. I have cooked a gammon over 3.5 kg and just increased the time to 10 hours.