You may need to soak the gammon joint overnight in cold water to remove any excess saltiness. To decide if this is necessary, cut a small piece of gammon from the joint and fry it until thoroughly cooked and then taste. If you think it tastes too salty, place the gammon joint in a bowl of cold water, cover and place in the fridge overnight. Discard the soaking water and rinse the joint in fresh cold water.
Preheat the oven to 170°C fan. Place all the ingredients including the gammon joint in a large casserole dish, stir to combine and cover with a tight-fitting lid.
Place in the oven and cook for 2½ - 3 hours, stirring every 30 minutes and checking that the stew isn't drying out. If it is, add a splash of water and stir in. The stew will be ready when the gammon is tender, falls apart and the sauce has thickened. Use two forks to shred the gammon and mix in. If the stew is not thick enough, remove the lid and continue to cook for a few minutes longer. Serve.