Put the water and salt in a large pan and bring to the boil. Crumble in a stock cube of your choice. While the water is heating, peel and dice the carrots, rinse them in cold water and add them to the pan. Peel and dice the potatoes, allow them to soak in fresh water for two minutes to remove any excess starch, then add them to the pan too. Rinse the leeks, cut off the green leaves at the top and the root at the bottom. Slice the remaining white stem into round slices about ½'" (1cm) thick. Add these to the pan then cover and simmer for 15 minutes or until the vegetables are tender.