Heat the oil in a large, heavy-based saucepan. Add the bacon and cook for 5 minutes until crisp, then remove from the pan and set aside. Add the butter and leeks to the pan and cook for 3–5 minutes until softened.
Stir in the flour, then gradually add the stock and milk, stirring continuously to avoid any lumps forming. Bring to the boil, add the potatoes and simmer for 7 minutes.
Arrange the cod fillets in the pan and cook for a further 7 minutes until they are cooked through. Transfer the cod to warmed shallow serving bowls.
Stir the cream and most of the bacon into the sauce and season to taste. Warm through and ladle around the fish. Serve sprinkled with chives and the remaining bacon.