Rinse the fish bones and trimmings of any blood, which would make the stock look cloudy and taste bitter. Chop the leeks roughly and place into a large heavy-bottomed stockpot.
Chop the fennel and carrots roughly and add to the stockpot.
Roughly chop the parsley, including the stems, and add to the stockpot.
Add the fish trimmings to the stockpot.
Pour in the white wine, then add enough cold water to cover the fish and vegetables (about 1 litre/2 pints).
Place the stockpot onto a high heat and bring the liquid to a simmer. After five minutes, remove the scum that forms on the surface with a spoon and discard. Reduce the heat and simmer, covered, for about 25 minutes, skimming as necessary.
At the end of cooking time, remove the stock from the heat and strain, discarding the fish trimmings and the vegetables.
Cool and store in the fridge for up to three days or freeze. Alternatively you can reduce the stock further and then freeze it in ice cube trays (freezing it this way means you can defrost as little or as much as you need at a time).