Butter a 1.5 litre baking dish. Butter the bread on one side and cut into triangles.
Layer half the bread in the dish, scatter over the sultanas and zest, and add the rest of the bread.
Warm the milk and cream, whisk the eggs, sugar, vanilla and nutmeg, then whisk in the warm milk mixture.
Pour the custard over the bread and leave to soak for 20 to 30 minutes.
Sprinkle with demerara sugar.
Bake at 180°C (160°C fan, gas 4) for 35 to 40 minutes until just set and golden. Rest for 10 minutes before serving.