Heat oven to 140C/fan 120C/gas 1. Stir the custard together with the milk. Trim the crusts from the bread, cut into triangles, then place in a large bowl with the raisins or dried cherries. Pour over the custard mixture, then carefully stir everything together so all the pieces of bread are coated. Lightly grease a small ovenproof dish with butter, then spoon in the mixture.
Cook for 30-35 mins until there is just a slight wobble in the centre of the custard. Sprinkle over the sugar to cover the surface, then pop under a hot grill for 1-2 mins until the sugar starts to melt and caramelise.