Put the currants into a saucepan with 250 millilitres of water and simmer for 5 minutes, then leave to rest in the water while you prepare the rest.
Generously butter the inside of the pudding dish and prepare the lid by laying a piece of foil on top of a piece of baking paper. Fold the two in half, then leave a 3 centimetre seam and fold them back on themselves. With the foil on top, cut out a circle large enough for the top of the pudding bowl with a 5 centimetre edge all of the way around.
Mix the self raising flour, sugar and salt in a mixing bowl then add the suet and grate in the lemon zest. Mix well then add the milk and stir to combine - adding another tablespoon or so of milk if it feels very thick. Drain the currants, discarding the water and mix through before pouring the batter into the prepared pudding dish. Butter the baking paper side of the lid, and place it face down on top of the dish. Use a piece of string to tie a tight seal underneath the lip of the pudding bowl. Create a handle from one side of the bowl to the other by tying a piece of string to the tight string under the lip.
Put a clean folded J cloth or tea towel in the bottom of a large saucepan to stop the dish clattering around, and sit the pudding bowl on top. Fill the pan with boiling water, two-thirds of the way up the pudding bowl. Cover with a lid and keep the pan simmering for 1 hour 30 minutes, topping up with water as and when needed.
Once cooked, use the string handle to carefully lift the pudding from the water and leave it to rest for 5 minutes before turning out onto a plate and serving with lashings of custard.