Place metal cookie cutters, a folded towel, or crumpled tin foil in the bottom of a large stock pot to prevent the pudding mold from touching the bottom of the pot. Add water and bring to a boil. Generously grease a pudding mold.
Place the flour, sugar, baking powder, salt and suet (or butter) in a food processor and pulse until the mixture resembles coarse sand. Transfer the mixture to a large mixing bowl.
Add the milk, lemon zest, vanilla extract and currants and stir until combined. Scoop the batter into the prepared pudding mold and secure it tightly with the lid.
Lower the pudding mold into the boiling water so that the water comes up to the halfway point of the pudding mold. Reduce the heat to a simmer and simmer undisturbed for 90 minutes. You'll likely need to weight the pudding mold down with something heavy to keep it from bobbing up and down in the water. I use a plate and then put something heavy on top of it, like another bowl. Check the water level periodically and add more water as needed to keep it at the halfway point of the pudding tin.
Remove the pudding mold from the water and let it sit for 15 minutes. Invert the pudding onto a plate. Slice into wedges and serve warm with English Custard Sauce.
See NOTE about reheating.
*Caster sugar is simply granulated sugar that has been very finely ground. It is the most popular form of sugar in British baking and is especially suited to items with a short baking time or delicate items like meringue. Because it's so finely ground it dissolves more quickly. However, after steaming this pudding for 90 minutes, regular granulated sugar will definitely be dissolved. The choice is yours: Buy caster sugar, grind your own using a coffee or spice grinder, or simply use regular granulated sugar.* REHEATING: The key is just to do it in a way that doesn’t dry out the pudding or render it overly moist and mushy. Two methods: Reheat the same way you cooked it by removing the wrapping and returning the the pudding to the mold to steam it for 3/4 to 1 hour or until hot. Or reheat it in the oven: Wrap the pudding in foil and reheat at 300ºF (150ºC) for an hour or until hot.