Put all the ingredients into a very large pan. Bring to the boil, then cover and simmer for about 1 hour, skimming the fat from time to time.
Strain through a fine sieve and discard all but the stock, then strain again, lining the sieve with muslin to achieve a clear liquid. Cool and refrigerate, then skim off any fat from the surface. Use within 2 days, or freeze and keep for up to 3 m