Preheat the oven to 180°C, fan 160°C, gas 4.
Spread the bread with the butter, cut into triangles and arrange in a buttered, 2-litre ovenproof dish. Scatter over the sultanas and set aside.
In a large jug, whisk together the milk, double cream, eggs, caster sugar and orange zest. Pour over the bread and set aside for at least 10 minutes. Scatter over the demerara sugar and bake for 30-35 minutes until golden.
Cook's tip: give it an international flavour by simply switching the bread. Give it a French twist by using slices of brioche or an Italian flavour by using panettone.