Combine the flour, suet, currants, raisins, sugar, and salt in a large mixing bowl, tossing everything together with your hands. The suet should be evenly distributed — no clumps hiding in corners.
Pour the water in slowly while stirring with a wooden spoon, bringing the mixture together into a soft dough that holds its shape without being sticky. If it feels too dry, add water a tablespoon at a time until it comes together.
Form the dough into a compact log about 8 inches long, then wrap it snugly in greased parchment paper, twisting the ends like a candy wrapper. Wrap the whole thing in foil for a second seal — this prevents water from seeping in during steaming.
Set the wrapped pudding on a steamer rack or heatproof plate inside a large pot with a tight-fitting lid. Add enough boiling water to reach halfway up the sides of the pudding, then cover the pot completely.
Keep the water at a gentle boil and steam for exactly 2 hours, topping off with more boiling water every 30 minutes to maintain the level. The pudding is done when it feels firm to the touch through the wrapping.
⏱ 2 hrs
Carefully unwrap the hot pudding and transfer to a serving plate. Slice into thick rounds while still warm — the texture is best when it's fresh from the steamer.