Rinse the fish bones and skin and place in a stockpot or large heavy based saucepan.
Chop the leeks, carrots and celery roughly and add to the stockpot.
Add everything else to the stockpot, then add enough cold water to cover the fish and vegetables (about 1.5 litre/6 cups).
Place the stockpot onto a high heat and bring the liquid to a simmer.
After five minutes, start to remove the scum that forms on the surface with a slotted spoon or skimmer and discard. Reduce the heat and simmer on low heat, covered, for 25 minutes.
At the end of cooking time, strain the stock into a clean pot, discarding all the solid contents. Cool to room temperature, then store in the fridge for up to 3 days, or freeze.