Preheat oven to 375 degrees F. Place pumpkin pieces skin side up and close together in a large baking dish. Pour over 1/2 cup water and bake for about 1 hour, or until a fork pierces the flesh easily.Melt half the butter and gently saute leeks until softened. Remove and set aside. Add remaining butter and cook onion and potatoes until golden. Stir in milk and simmer for 20 minutes. Don't worry if the milk reduces; just make sure that there is enough liquid to prevent sticking to the pan.When pumpkin is tender and golden, remove from oven, cool, then remove skin and any excessively browned surfaces. In the meantime, put chicken stock on to boil, add orzo and cook until barely done. Remove pasta with a slotted spoon and set aside with leeks. Reserve stock. Now blend pumpkin and potato mixture in a food processor or force it through a fine sieve. Season with salt, pepper and cayenne to taste.Transfer to a clean saucepan and blend in the hot stock. Add cream, bring to the boil, and then stir in leeks and pasta. If the soup is too thick, thin it with extra stock or water. Adjust the seasoning and stir in mint.