Lightly butter a baking dish around 10in x 7in (25cm x 18cm).
Cut the hot cross buns into slices from top to bottom, around ½in/1.2cm thick. I got around 4 slices per bun, but it will depend a little on the size of the buns.
Butter the slices of hot cross bun on one side, relatively generously, and then top with a thin spread of marmalade. Layer the slices in the baking dish, with the slices slightly overlapping each other so that they fill the dish. It can be a bit of a jigsaw puzzle to get them to fit, so you might have some one way, some another - I usually find some end pieces in first are best as they allow it to overlap better. Tuck the chocolate chips in between the slices so they are relatively evenly spread.
Place the milk, cream, eggs, vanilla and sugar in a bowl (ideally with pouring spout) or mixing jug and mix everything so well combined. Pour this custard mixture evenly over the bun slices in the dish, making sure you pour some on each slice.
Leave the hot cross buns to soak up the custard a little, at least 20 minutes but a little longer is also fine. If longer than 20 minutes, place in the fridge. You can also cover and leave in the fridge overnight if it suits your schedule better. Press the slices down under the custard a little, then put back in place, as needed, to help them soak.
Preheat the oven to 350F/180C towards the end of the time the pudding is soaking.
Once the oven is heated, sprinkle the demerara sugar evenly over the top then bake until the top is gently golden and slightly crisp and the custard mixture has set, around 35 minutes. Leave it to rest around 5 minutes before serving to help the custard fully set. Best enjoyed warm.