Clean the vegetables then put into a large saucepan. Add 6-8 cups of cold water. Bring slowly to the boil. Cook gently until the vegetables are tender. Strain broth into a bowl then add a little salt and Marmite. Set aside to cool.
Return the vegetables to the saucepan with 4-5 cups of cold water. Boil again then strain again. Discard the vegetables then add the old stock to the new stock.