Heat the oven to 200C/400F/Gas 6.
Put the bones in a roasting tin and bake until well browned.
Put the carrot, onion and celery in another roasting tin and toss in the oil. Bake until well browned.
Put the vegetables and the bones in a large pan and add the peppercorns, bay leaves, parsley and thyme. Cover with water.
Bring to the boil and skim off any scum that has formed. Cover and simmer very gently for 3-4 hours. From time to time, skim off any scum that forms. 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 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.