Wash the chicken bones under cold running water to remove any excess blood, then drain and put aside. Sweat the vegetables in a large stockpot in the grapeseed oil. Add the chicken bones and lightly sauté, being careful not to add too much colour. Add 8 litres (270 fl oz) of cold water and bring to the boil. As soon as it boils, turn down the heat to achieve a very gentle simmer (the stock should be just ticking over). Skim the stock with a ladle and continue to simmer for 4 hours. After 4 hours, skim the stock again with a ladle. Strain the stock carefully through a chinois or fine strainer. Allow the stock to cool completely and refrigerate until required.