Mix the flour, suet, currants, sugar and salt in a large bowl. Stir in the milk to a soft, sticky dough, adding a splash more if it looks dry.
Turn onto a floured surface and shape into a fat sausage about 20 cm long. Lay it on a large sheet of baking paper and roll it up loosely, twisting the ends like a cracker so the pudding has room to swell.
Rest the parcel on a folded sheet of foil as a sling and steam it over simmering water in a covered pan for 1 hour 30 minutes, topping up the hot water now and then.
For the custard, blend the custard powder and sugar with 3 tablespoons of the milk into a smooth paste. Heat the rest of the milk until steaming, then stir in the paste and simmer gently, stirring, until it thickens enough to coat the spoon.
Unwrap the pudding, rest for 5 minutes, then slice into 6 rounds with a bread knife. Pour over the hot custard.