Put all the ingredients into a large pan and cover with water. Bring to the boil. Cover and simmer very gently for 20-30 minutes. Strain into a large bowl and allow to cool.
Use within 3 days or freeze. To freeze, reduce the stock by half by boiling vigorously and cool. Pour into ice-cube trays and freeze. When frozen put the cubes into a bag, label it, and when you want to use one, just put in a jug and add boiling water to dissolve the cube.