Use a knife to score a diagonal pattern onto the ham with the lines about 2.5cm apart. Place the ham in a large roasting tin and pat dark brown soft sugar over the entire surface. Some will fall into the tin, that's okay.
Bake the ham in the preheated oven until the sugar is melting off of it, about 30 minutes. Remove the ham from the oven and pour the cola, grape juice and juice from the pineapple into the tin. Baste the ham with the mixture and return it to the oven.
Continue to bake in the preheated oven, basting every 20 minutes, for about 2 hours. Cut the pineapple rings in half. Remove the ham from the oven and float the pineapple rings in the drippings. Allow the ham to rest for about 10 minutes, then slice and serve with the pineapple and sauce.