Remove the crusts from the bread and using the remaining butter, butter both sides, then cut each slice into quarters.
Arrange a single layer of the bread triangles, slightly overlapping in the bottom of the buttered dish.
Scatter over some of the sultanas and place another layer of the bread triangles on top and scatter over the remaining raisins. Press down gently with a fish slice or spatula.
To make the custard, heat the cream and milk in a pan until it almost comes to the boil. Remove from the heat. Meanwhile, whisk together the eggs and sugar in a large heatproof bowl set over a pan of simmering water until thickened and the whisk leaves a trail in the mixture.
Remove from the heat and beat in the cream mixture until well combined.
Pour two-thirds of the custard over the layered-up bread triangles and leave to stand for about 30 minutes or until the bread has soaked up all of the custard.
Preheat the oven to 180°C (350°F/Gas 4).
Pour the remaining custard over the soaked bread and butter triangles and arrange the rest of the bread triangles on top.
Press down firmly with a fish slice so that the custard comes halfway up the bread triangles.
Bake for 30-35 minutes until the custard is just set and the top is golden brown.