Break the hot cross buns into small pieces and add to a mixing bowl. Pour over the milk and soak for 30 minutes.
Pre-heat the oven to 180C/Gas 4.
Grease a 12x6" baking tin, or line with greaseproof paper.
Beat the hot cross bun and milk mixture together with a fork, then add the fruit, brown sugar, suet, beaten egg, and spice. Mix well to combine.
Pour into the prepared tin and spread evenly, making sure all of the corners are filled.
Grate nutmeg over the top, to taste.
Bake in the centre of the oven for 1 hour 10 minutes (or 1 hour 30 minutes if not making the crosses).
To make the crosses, whisk 75g flour with the water, adding one tablespoon at a time. Once combined, add the mixture to a piping bag with a thin nozzle and pipe over the bread pudding after it’s been cooking for 1 hour 10 minutes. (Alternatively, add the mixture to a zip lock bag and cut the corner off in place of a piping bag and nozzle.) Return to the oven for 20 minutes and cook at 200C/Gas 5.
Once cooked (a knife will come out clean, when inserted into the pudding), sprinkle white granulated sugar over the top and leave to cool in the tin.