Butter a 1.2–1.5 litre pudding basin or bowl: grease the inside thoroughly with softened Butter.
In a large mixing bowl, combine the Self-Raising Flour, shredded Shredded Beef Suet, Caster Sugar, Currants, Lemon Zest and a pinch of Salt; toss together so the dried ingredients are evenly distributed.
Gradually stir in enough Whole Milk to bring the mixture to a soft, dropping consistency — it should be moist but not runny.
Spoon the mixture into the prepared basin, pressing down lightly to remove large air pockets and level the top; you can dot a little extra Butter on top if you like.
Cover the basin: place a circle of greaseproof paper or baking parchment on top and secure with foil, or fit a lid if your basin has one; tie a piece of string around the rim to keep the cover secure if needed.
Place the basin in a large saucepan and pour in boiling water until it comes about halfway up the sides of the basin. Bring to a gentle simmer, cover the pan and steam for about 1½–2 hours, topping up the water if it gets low; a skewer inserted into the center should come out clean when done.
Carefully lift the hot basin from the pan, remove the cover and let the pudding rest for a few minutes before turning out onto a serving plate.
Slice and serve warm with plenty of hot Custard spooned over each portion.
