Cook the leek and onion in a large pan with the oil over a medium heat for around 5 minutes, or until softened.
Standing the corn cob on its end, remove the kernels using a sharp knife, cutting down the length of the cob. Add the sweetcorn kernels, potatoes and stock to the pan and bring to the boil and simmer for 10-15 minutes, until the potato and sweetcorn are tender.
Add the pieces of haddock (check for bones) to the soup along with the milk, and cook for a further 5 minutes until the fish is cooked through.
Stir through the chopped parsley before serving and then spoon the chowder into bowls or mugs.