Pour 2 tbsp whiskey over sultanas and leave to plump up - about 15 minutes.
Butter bread liberally and cut into triangles. Arrange in a rooftile pattern, overlapping in a shallow ovenproof dish. Pour remaining whiskey over bread and allow to soak in - about 5 minutes. Scatter sultanas and soaking whiskey over as well.
Beat eggs and egg yolks together with 1 tbsp sugar. Add cream. Pour over bread. Sprinkle remaining sugar over the top.
Bake in a pre-heated oven Gas 5 375F 190C for around 30 minutes, or until custard is set, bread is sugar crusted and points are crisply browned.