Prepare the vegetables. Chop the potatoes into evenly sized pieces, about 2cm square. Slice the leeks.
Put the leeks and potatoes into a large saucepan and add water or stock. Bring the water or stock to the boil and then simmer (for approximately 15 mins) until the potatoes and leeks are cooked.
Take the saucepan off the heat and whilst still warm, use a hand blender to blend the soup to your preferred consistency. If using milk, add at this stage to your taste and mix in thoroughly.
Add, salt, pepper and, if using, a pinch of mace to your taste. Reheat the soup if needed.
Ladle the soup into bowls. Before serving, if using, add a dollop of double or sour cream.