Butter the cut side of the bottom half of the brioche with half the butter, then spread over five tablespoons of the marmalade. Put the other half of the loaf on top and cut it into sandwiches, then cut each sandwich into two triangles.
Butter the tops of the triangles, then put them into a baking dish that comfortably fits them.
Heat the oven to 180C (160C fan)/gas 4. Whisk the eggs and sugar, then whisk in the whisky, milk, cream and remaining marmalade. Grate in the nutmeg, lemon and orange zest.
Pour the custard over the brioche sandwiches, pushing the bread down into the liquid so that it is completely covered. Leave to rest for 30 minutes.
Scatter the demerara sugar over the top of the pudding and bake for 35-40 minutes, until golden and puffed up. Leave to cool for 10 minutes before serving with cream or creme fraiche.