I usually use half demerara and half granulated white.
Always use granulated sugar in any recipe when it says Caster sugar, never had a problem.
My bread pudding recipe I have done for years and passed down the generations;
1/2 loaf of bread
4oz mixed fruit (I use currants, diced apple, raspberries, grapes in any combination)
2 tsp mixed spice
4oz margarine
1 egg beaten
1 tbsp syrup
2oz sugar
Soak 1/2 loaf in water for a good few hours.
Squeeze out water with a sieve and put bread in a bowl.
Add mixed fruit, spice and sugar, mix well.
Add beaten egg, mix well.
Stir in the syrup.
If you find it too soggy, you can add a little flour.
Test with a skewer, should come out clean.
Cook in a loaf tin for 3/4 hour to 1 hour at 180c.