Leek & potato soup

Name: Leek & potato soup
Prep time: 20 min
Cook time: 25 min
Total time: 45 min
Cuisine: British
Ingredients: 50g butter; 450g potatoes,peeled and cut into 1cm pieces (try Golden Wonders or Kerr Pinks); 1 small onion,cut the same size as the potatoes; 450g white parts of leeks,sliced (save the green tops for another soup or stock); 850ml-1.2litres/1.5-2pts light chicken or vegetable stock; 142ml carton whipping cream; 125ml full-fat milk; the white part of 1 leek; a small knob of butter; finely chopped chives