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.
Place all the ingredients including the gammon joint in the Instant Pot, stir to combine and cover with the lid.
Lock the lid in place and set the vent to 'sealing'. Set the pressure to 'high' and cook for 45 minutes. Allow the pressure to release naturally and when the valve has dropped, open the lid.
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, set to 'sauté' and continue to cook for a few minutes longer. Serve.