You will need a 28cm (11in) wide-based round ovenproof dish, and a roasting tin large enough to place it in.
Butter your round ovenproof dish. Cut the brioche loaf into 1.5cm (⅝in) slices and brush each slice with melted butter on both sides. Arrange the slices in a spiral in the buttered dish and scatter over the sultanas.
To make the custard, place all the ingredients in a large bowl and whisk together by hand until well mixed. Pour the custard over the brioche in the dish and gently press the exposed bread crusts down into the liquid. Sprinkle with the demerara sugar and leave to stand for about 30 minutes.
While the pudding is standing, preheat the oven to 180°C/160°C fan/Gas 4.
Half-fill the roasting tin with boiling water, and place the ovenproof dish in this. Bake in the oven for about 40 minutes until golden on top and puffed up. The layers will shrink down a little on cooling. Serve warm with cream.
