Cut each hot cross bun into 2 cm thick slices then spread a good amount of butter onto one side.
Grease & line a 2 lb loaf tin with baking parchment.
Next, place a third of the hot cross buns into the tin (butter side up), overlapping the slices if necessary. Sprinkle half the sultanas over the hot cross buns then pour over enough custard to completely cover the buns.
Repeat the previous step to make the second layer.
To finish, place the remaining hot cross bun slices on top then pour over the remaining custard. Leave to soak for an hour.If the custard starts to overflow, let the pudding soak for 15 minutes before adding in more. Preheat an oven to 160°c/140°c fan (320°f/284°f).
Sprinkle a generous amount of demerara sugar over the top of the pudding then place the loaf tin into a small roasting tin. Fill the roasting tin half full with boiling water then place the pudding in the oven.
Bake until the internal temperature of the pudding reaches 75°c/167°f. This will take 40 minutes - 1 hour. Check the pudding after 40 minutes & if the top is browning too much, loosely cover it in foil.
Once cooked, take the pudding out of the oven & leave to cool to room temperature (still in the water). Then once cool, take the tin out of the water & refrigerate the pudding for at least 4 hours.