Grease a pudding mold.
Place a steamer rack inside a large pot. Add enough water to come halfway up the size of the pudding mold (or bundt pan) and bring to a boil.
In a large bowl, combine flour, sugar, baking powder, salt and suet.
Using a pastry blender, cut the suet into the dry ingredients.
In a small bowl, combine milk, lemon zest and vanilla.
Add milk to flour mix and stir to combine. You may have to knead with your hands until you have a soft dough.
Add currants and mix well.
Transfer to greased pudding mold (or bundt pan) and spread in an even layer.
Place inside the boiling pot of water on top of the steamer rack.
Cover well and steam for 1 hour.
After 1 hour, check the water level. If it has evaporated too low, add some more water.
Continue to steam for another 30-45 minutes, until set.
Leave to cool for 15 minutes.
Turn mold over onto a plate.
Serve with custard.