Wash the potatoes then, leaving the skins on, cut them into 2cm cubes.
In a stock pot, add 1L water and over medium heat. Crumble in the stock cubes until dissolved. Add the potatoes and top up with water so that the potatoes are covered with about 5cm (2 in) Bring to the boil, then turn down the heat to a simmer.
Wash the leaks and dry. Cut along the length of the leek, leaving the bulb intact. Cut into slices.
Add the leek to the pot and simmer for 20 minutes.
Pour in the single cream and stir.
Mix the cornflour with 3 tablespoons cold water and add to the soup, a little at a time, continuously stirring as you go. Simmer until you reach the desired thickness.
Add salt and pepper to taste. You can add a knob or two of butter for a buttery, creamy taste.
I prefer mine without butter by my children love the buttery taste, so I pour my serving out first, then add the butter.