Prepare the Pastry: In a mixing bowl, combine the self-raising flour, suet, and salt. Gradually add cold water, a tablespoon at a time, until the mixture comes together into a soft but firm dough. Wrap in cling film and set aside.

Cook the Leeks: In a frying pan, melt the butter over medium heat. Add the sliced leeks and cook gently for 8-10 minutes until soft and tender. Season with salt and black pepper. Allow to cool slightly.

Roll Out the Pastry: Roll out the pastry on a floured surface to about 0.5cm thick. Reserve a small portion of the pastry for the lid.

Assemble the Pudding: Grease a pudding basin lightly with butter. Line the basin with the rolled-out pastry, pressing it gently into the sides and leaving an overhang. Fill the basin with the cooked leeks, then cover with the reserved pastry to create a lid. Seal the edges by pinching the dough together and trim any excess.

Prepare for Steaming: Cover the pudding basin with a layer of baking parchment and foil, pleated in the center to allow for expansion. Tie securely with string.

Steam the Pudding: Place the pudding basin in a steamer or a large saucepan with a steaming rack. Add enough boiling water to reach halfway up the sides of the basin. Cover with a lid and steam for 2 hours, topping up with boiling water as needed.

Serve and Enjoy: Carefully remove the pudding from the basin. Slice and serve hot as a side dish or a main course.