Heat the milk and cream gently in a saucepan with the vanilla pod until almost boiling. Remove from the heat, cover and leave to infuse for 30 minutes.
Meanwhile, spread each slice of bread liberally with butter and cut it in half diagonally. Overlap the slices attractively in a buttered, shallow, ovenproof dish, scattering sultanas over the pieces as you go.
Whisk the egg yolks and caster sugar in a large bowl until pale and creamy. Strain the infused milk and cream onto the egg mixture and whisk well. Pour evenly over the buttered fruit bread and leave to soak for 20 minutes.
Meanwhile, preheat the oven to 180 degrees C. Sprinkle the nutmeg and sugar evenly over the pudding and set the dish in a roasting pan. Pour in enough boiling water to come halfway up the side of the dish and bake in the centre of the oven for 45 minutes, or until the custard is just set and the top is golden and crisp.
Start to make the sauce 15 minutes before the pudding is due to come out of the oven. Put the sugar and egg yolks in a heatproof bowl and place snugly over a pan of barely simmering water, making sure that the bottom of the bowl does not touch the water. Whisk until the egg and sugar mixture is pale and creamy and the sugar has dissolved. Still whisking, slowly pour in the cider and continue to whisk for 5 minutes, or until the mixture froths up to a creamy mousse. Whisk for another 5–10 minutes, or until the sauce is light and foamy.
Pour the sauce into a jug and serve immediately with the hot pudding.