Dismember the duck carcass and chop it into 2.5cm pieces making sure all the bones are split. This releases the natural gelatine.
Put the chopped carcass into a large pan with the herbs. Cover with cold salted water and bring to a boil, reduce the heat, and simmer gently for 2 hours.
Strain the resulting stock into a smaller pan. Bring to a boil, reduce the heat and simmer until reduced by half.
Allow to cool a little, and pour into a suitable container, such as a pyrex bowl. Once at room temperature place into the fridge and leave overnight.
Remove from the fridge, you will now have a gel-style stock with a fat layer on top. Carefully remove the fat layer, this is great for the roasties.
Keep in the fridge or freeze for later use.