Add the water to the pressure cooker and steam the potatoes (in a basket) until they are tender - about 15 minutes.
Pour the cooking water into a bowl and add the stock cube cube. Reserve 1/4 of the potatoes as they are and blend or mash the rest of them mixing in all but 1/4 cup of the stock.
Meanwhile, cook the leeks in the pressure cooker for about 8 minutes with the remaining 1/4 cup of stock then add them and all the liquid to the potato mix.
Finally mix in the the white beans and reserved potatoes. If soup is too thick, you can add a little water. If it is too runny, you can crush a few pieces of potatoes and stir into the soup. Season to taste if needed then serve.