Finely slice and fry the leeks in the butter for a bit longer than 5 minutes until soft but not yet brown. Add the milk and potatoes and bring to the boil, simmering for 11-15 minutes(make sure the potatoes are cooked through). Stir thoroughly, take about a third of the contents and liquidise it before returning it to the rest of the chowder. Now chop the haddock into smallish bitesize pieces and add to the chowder, cooking for just 5 minutes before garnishing and serving with crusty bread.