Preheat the oven to 200°C.
Sift flour, salt, and brown sugar into a bowl.
Mix in the suet and enough water to form a soft dough.
Roll out the dough on a floured surface to a 20cm x 30cm rectangle.
Spread warmed jam over the pastry, leaving a 1cm border.
Fold the border in and brush it with milk.
Roll the pastry loosely from the short side and seal the ends.
Place the roll seam-side down on a greased baking sheet.
Brush with beaten egg and sprinkle with demerara sugar.
Bake for 35 to 40 minutes until golden brown.
Sprinkle with more sugar and serve hot with custard.