Place the fish in a wide saucepan along with the whole peppercorns and cover with the milk. Bring to a gentle simmer and switch the heat off, pop the lid on, and leave to stand for 10 minutes.
Meanwhile, heat the butter in a soup pan and gently cook the leeks, onion and bacon until the vegetables soften.
Add the peeled, cubed potatoes and cook for a further five minutes.
Strain the fish reserving the milk and discarding the peppercorns. Keep the fish warm (I pop it in a bowl and put a plate on top).
Add the milk to the potatoes and vegetables and simmer, gently, until the potatoes are tender. Don't worry if the milk curdles a bit.
Using a hand blender or potato masher, puree half of the mixture (a hand blender will give you a creamier texture) and return to the pan.
Add the double cream and flake and add the fish. Season to taste.
Reheat the soup but don't allow it to boil.
Serve immediately with a sprinkling of fresh or dried parsley.