Position a rack in the bottom third of the oven and preheat to 160°C/Gas Mark 3.
Trim any excess skin and/or fat from the ham. Using a sharp knife, score the ham in a diamond pattern, making 6mm deep cuts spaced 2.5cm apart. Place on the rack of a roasting pan
In a small bowl, combine the brown sugar and mustard. Rub the mixture all over the ham. Place the orange wedges on the roasting rack, around the edges of the ham. Pour the Coca-Cola into the bottom of the roasting pan (not over the ham). Cover the pan tightly with foil, wrapping it around the edges of the pan to seal. Using a small, sharp knife, make three small slits in the top of the foil.
Roast until the internal temperature reads 74°C, should take three to three and a half hours; after the first hour, baste with the pan juices every 30 minutes.
Cover the ham with foil and let the ham rest in the pan for 30 minutes. Baste once more with the Coke mixture at the bottom of the roasting pan, then transfer to a carving board and slice.