Put the fish trimmings in a large sieve, rinse with cold water, drain and then put them into a large saucepan with all the remaining ingredients.
Bring slowly to the boil. Skim off any scum with a slotted spoon. Cover and simmer for 30 minutes.
Strain the stock through a fine sieve, return it to the saucepan and then simmer it, uncovered, for about 15 minutes until reduced by half. Cool then chill in the refrigerator for up to 3 days.
If you are adding fish heads, don’t cook them for longer than 30 minutes before straining, or they will begin to add a bitter taste.