Heat the milk to a medium simmer, then add the smoked haddock to the pan, in large pieces, skin side down. Simmer for four minutes, then remove from heat and allow to remain in the milk a further 2 minutes (6 minutes, total cooking time). Pour the milk into a jug.

When the fish is cooked, remove the skin. Keep the fish in large flakes.

Melt 1 ounce (1/4 stick) butter in a small pot, then add the flour. Stir and cook for a few minutes, but do not allow to brown. Start adding the reserved milk a little at a time, creating a sauce, (keeping a third of the milk set aside.) Continue adding the milk until a thick, yet runny sauce is reached.

Add the turmeric, English mustard powder, (cayenne, if using) and pepper. Stir well.

In another pan, sautée the leek in the remaining 1 ounce of butter, then add to the sauce. Add the flaked smoked haddock, including any juice that has accumulated.

Stir over medium low heat until heated through. Adjust the sauce with the reserved milk to a desired consistency and serve as desired.