Pin-bone and skin the smoked haddock fillet, and cut into thick chunks. Put the haddock into a large saucepan, pour over the milk and add the bayleaf. Bring slowly to the boil and reduce the temperature to poach over a low heat for 3 – 4 minutes. Remove the fish from the milk and strain the milk ready for use in the sauce.
Cook the macaroni in boiling salted water for 7 minutes, then add the cauliflower florets and continue to simmer until the pasta is just cooked and the cauliflower tender. Strain and allow to drain for a few minutes.
In a medium sized saucepan, cook the spring onions in the butter for 3 – 4 minutes or until tender. Blend in the flour with the dried mustard and cayenne pepper. Cook over a very low heat for 1 minute then blend in the poaching milk, whisking to prevent lumps forming. Bring to the boil and simmer for 2 minutes. Stir in the grainy mustard and most of the Cheddar cheese and season to taste with salt and pepper.
Heat the oven to gas mark 6/200°C/fan 180°C. Mix the smoked haddock, cauliflower and macaroni together and tip into a large gratin dish. Spoon the cheese sauce over the top and sprinkle over the remaining cheese. Bake on the top shelf of the oven for 15 - 20 minutes until the top is golden brown and it is piping hot.