In a mixing bowl, combine the self-rising flour, shredded suet, granulated sugar, a pinch of salt, currants or raisins, and lemon zest.
Gradually add the milk, mixing until you have a soft, sticky dough.
Grease a pudding basin with butter and spoon the dough into it.
Cover the basin with a double layer of parchment paper or foil, securing it with string.
Steam the pudding for about 2 hours. Ensure the water level in the steamer is topped up as needed.
While the pudding steams, prepare the custard by mixing custard powder and granulated sugar in a saucepan, then gradually adding the whole milk. Heat gently, stirring constantly, until the custard thickens.
Once steamed, remove the pudding from the basin, slice, and serve with warm custard.