You will need a 1.2 litre pudding basin, a large saucepan with a lid, a saucer, baking parchment, tin foil and string (unless your pudding basin has it’s own lid).
Start by thoroughly greasing a 1.2 litre pudding basin. Place a disc of baking parchment in the bottom to prevent the top of the pudding from sticking.
Pop the kettle on. Cut a large square of baking parchment and another of tin foil. Make a pleat in the centre of each square. The pleat gives the pudding room to rise. (If you have a pudding basin with a lid you can skip this step.)
Mix together the flour, baking powder and salt in a large bowl.
Add the vegetable suet, cold block butter or cold coconut oil. Rub it in using your fingertips until the mixture resembles fine breadcrumbs. You can also do this in a food processor to make things easier. (I recommend using the food processor!)
Stir through the sugar, currants and lemon zest then add the milk and mix to form a firm but moist dough.
Transfer the dough to the pudding basin and use the back of a wet spoon to spread it level. There should be a decent gap at the top to allow the pudding plenty of room to rise.
Place the baking parchment on top of the pudding basin with the pleat in the middle, followed by the tin foil. Press the foil over the edge of the pudding basin.
Tie a length of string very tightly several times around the foil – there should be a lip on the pudding basin; tie the string just under that.
Tie a length of string across the top of the basin to make a handle. Trim off the excess foil and parchment, leaving at least a centimetre below the string. If your pudding basin has it’s own lid, skip the above and simply put the lid on, making sure it is tight.
Place a saucer upside down in a large saucepan with a lid and place the pudding basin on top. Pour boiling water into the pan so that it comes about halfway up the pudding basin. Do not let the water touch the foil/baking parchment otherwise you may end up with a soggy pudding.
Put the lid on the pan and place over a low heat. Cook at a gentle simmer for 1 ½ hours, it will do the pudding no harm to let it cook for a while longer however. Check after an hour to see if the water needs topping up but resist the urge to take the lid off the pan too often.
Once the pudding is done steaming, cut off the string and remove the tin foil and parchment. Run a knife around the edge to loosen it then place a serving platter on top and flip it over. The pudding should drop out of the basin (you might need to give it a bit of a wiggle).
Serve right away with custard.