Put the flour, salt, suet and the sugar in a bowl. Stir in enough of the measurement water to mix to a soft but not sticky dough. Turn the dough out on to a floured surface and roll out to a rectangle about 19 cm x 15 cm (7 ½ inches x 6 inches) and about 5 mm (¼ inch) thick.
Spread 3 tablespoons of the jam over the dough to about 1 cm (½ inch) away from the edges. Brush the edges with milk and roll up the dough, starting from a short end. Pinch the ends firmly together to seal. Wrap loosely in greased foil and place on the upper rack of the halogen oven. Brush with a little more milk and sprinkle with extra sugar.
Pour boiling water to a depth of 3 cm (1¼ inches) into the bowl of the halogen oven. Set the temperature to 150°C (302°F) and cook the roly-poly for 1 hour until golden, covering with foil if the top starts to brown too much. Transfer to a serving plate.
Warm the remaining jam in a small saucepan and drizzle over the pudding. Serve with vanilla custard.