Put all the ingredients into a large pan and cover with water. Bring to the boil and skim off any scum that has formed. Cover and simmer very gently for 2-3 hours.
Strain into a large bowl and allow to cool. Chill overnight.
Skim off any fat that has formed on the surface. Use within 3 days or freeze.
To freeze, reduce the stock by half by boiling vigorously. 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.