Sift the flour, salt, sugar and baking powder into a bowl, then stir in the suet.
Gradually beat in the water to form a light dough.
Add a little more water if necessary.
Roll out to a rectangle about .75 cm./1/4 in.thick.
Sprinkle the sultanas over the dough, then brush the edges with the milk.
Roll up, like a Swiss roll, and seal the edges.
Enclose the roly poly in foil, leaving room for expansion.
Put it into a large pan half-filled with hot water and steam over low heat for 2 1/2 hours, or until a knife inserted into the centre of the pudding comes out clean.