Break up the carcass into several pieces and put in a large saucepan or stockpot. Add 2 litres of cold water and bring to the boil. Skim off any scum that rises to the surface, then add the bouquet garni, peppercorns and salt.
Put the chicken, giblets and water in a stockpot or saucepan and bring slowly to the boil. Add a cup of cold water and reduce the heat. Skim any scum from the surface, then add the prepared vegetables, herbs and peppercorns. Simmer over low heat for 5 hours, skimming the scum frequently from the surface of the stock.
Partially cover the pan with a lid, reduce the heat and simmer gently for about 2 hours, or until the stock has reduced by half.
Remove the pan from the heat and strain the stock, discarding the bones and vegetables. Leave to cool, then skim off the fat that has risen to the surface.
Store in a plastic container in the fridge for up to 4 days or freeze for up to 6 months.