Mix raisins, sultanas, rum, and 200ml water in a bowl and soak for at least four hours.
Combine remaining pudding ingredients with the soaked fruit and liquid to form a soft dough.
Shape the dough into a log, wrap in foil, then muslin, and secure the ends with string.
Simmer the pudding in a pan on a heatproof plate covered with water for two hours.
Rest the pudding for five minutes before removing the muslin and foil.
Whisk egg yolks and sugar together until the mixture is thick and pale.
Boil milk and cream, pour onto the eggs, then heat slowly in a pan until thickened.
Slice the pudding thickly and serve with the custard ladled over the top.