Remove the green outer leaves from the cauliflower as well as the thick white stalk, taking care not to break off the florets. Cut a deep cross in the bottom of the stem and steam it whole over a pan of boiling water until slightly tender but not cooked through. The cauliflower florets should still be firm. Remove the cauliflower from the heat and leave to cool.
Place the butter in a large saucepan and melt over medium heat. Lower the heat and add the finely chopped leeks. Cook until the leeks are softening slightly but not cooked thoroughly. Remove 1 tablespoon of the leeks and keep to one side. Add the flour to the remaining leeks in the pan and stir. The flour will become lumpy, but continue to stir for 2 - 3 minutes. This will cook the flour and soften it to make a better sauce. Add the large pinch of salt and the mustard powder and stir again. Turn up the heat to medium and add the milk all at once. Stir briskly until a smooth sauce forms. Continue stirring until the sauce is thick and glossy - about 4 - 5 minutes. If the sauce thickens too much, add a little more milk. It should be thick, but still a bit runny. At this point, add the grated cheese and stir until melted. Remove from heat.
Break or cut the cauliflower florets away from the thick, central stalk, taking care not to break it into tiny pieces. Place these florets in a greased baking dish large enough to hold all the florets in just one layer. Pour the cheese and leek sauce over the cauliflower making sure to cover all the florets. Sprinkle the extra grated cheese evenly over the top and scatter the reserved softened leeks over the top as well. Finally, grate some fresh pepper over the top.
Bake in the preheated oven until the sauce is bubbling and golden brown on the top, approximately 20 minutes. Test the cauliflower to make sure it's cooked through by piercing with a sharp pointed knife, cook a little longer if necessary. Serve warm.