Make potato pedestals by peeling both potatoes, cutting each in half length ways, then cut off one end of each half so that the potato stands up vertically. These four halves will be used as stands for roasting the chickens.
Wash and pat dry the chicken. Season with salt and pepper inside and out. Add one grape inside of each.
Wrap each chicken with a rasher of bacon diagonally from the top of one breast back to the opposite leg, like a sash. Mount each chicken on top of the potato stand securing with a skewer. Place upright on a roasting dish. Brush the outside of the chicken with melted butter.
Roast for 1 hour in the preheated oven until juices run clear or the internal temperature reaches 80 degrees C. Add the rest of grapes to the roasting dish for the last 10 minutes of roasting.
Take out of the oven and let stand 10 minutes before serving.