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