Trim the tops and roots of the leeks, discard outer layer, slice finely, wash them thoroughly and drain well
Melt the butter in a large saucepan, add the sliced leeks and diced potatoes
Season with salt and pepper, then cover and cook on low heat for about 15 minutes
Add the stock and simmer gently for another 15 minutes or until the vegetables are soft
Bend to a puree in a liquidiser or press through a sieve
Reheat in a saucepan to serve, swirl in cream if liked and scatter chives or parsley on the top