Choose a deep pudding dish. Butter the slices of bread and then layer with each layer having a sprinkling of the rum, sugar, orange rind and juice and sultanas.
Prepare the custard but do not cook it until thickened. Pour over the bread. You will need more or less custard depending on your dish but it must come near the top.
Sprinkle over the remaining sugar then bake at 180 C, Gas 4 for 30 minutes or until just set. For a crunchy top finish under the grill.