Remove crusts from bread, butter each slice with the soft butter, then cut the slices in half diagonally to make each slice into two triangles. Arrange the bread in tiled layers (or just plain layers) in a 2 pint (4 cup / 1 litre) ovenproof dish. Sprinkle each layer with some currants and raisins. The top layer should be just bread; don't sprinkle the final layer with the dried fruit as the fruit will just burn.
Pour the custard over the bread in the ovenproof dish, and let it soak for 20 minutes. Then put the dish in the oven and bake for 20 minutes. Take the pudding out of the oven, sprinkle it with the sugar and put it back in the oven. Raise the temperature to a high heat, almost broiling (or grilling) and watch it until the sugar begins to carmelize. Remove and serve piping hot.
You can adjust the amount of custard. For instance, if you only have fresh bread to hand, you will likely want to use less custard so that the mixture doesn't stay too soggy. If your bread is very dry, you may want to use more.
The custard can be homemade, made from powder, or ready-to-serve from a carton. If you are making it from scratch, have it made before you start this recipe.