Butter a 6-cup pudding mold or basin. Pour 1 cup boiling water over the chopped raisins, add the margarine and stir until melted.
Mix the flour, sugar, bicarb soda and salt in a medium bowl. Stir in raisin mixture, golden syrup and egg. Pour into pudding mold. Cover with aluminium foil. Place the mold on a rack in a steamer and pour in boiling water to come halfway up the mold. Cover and simmer over low heat for 2 hours or until a skewer tests clean. Remove from the steamer and let stand five minutes before unmolding.
To make the rum sauce: beat the butter with an electric mixer about five minutes or until smooth. Gradually beat in the icing sugar and rum. Cover and refrigerate one hour before serving with the pudding.