Grease the baking dish with butter and set aside. Don't preheat the oven just yet (the bread and butter pudding needs to stand for 30 minutes before baking). The pudding will be baked at 170°C (conventional, not fan).
Place the cranberries, orange zest and juice, and the heaped tsp of caster sugar into a small saucepan over medium heat. Stirring frequently, bring to the boil, then cook for a further 30 seconds before removing from the heat and setting aside to cool.
Spread the bread slices thickly with the butter, then cut into your desired shapes.
Arrange half of the bread pieces, butter-side up, in the greased baking dish. Sprinkle over half of the remaining sugar, along with all of the cooled, soaked cranberries (and any liquid). Arrange the remaining bread pieces on top, butter-side up, sprinkle over the remaining sugar, then set aside while you make the custard.
For the custard, whisk the eggs together in a bowl until combined. Add the milk and cream, then whisk well. Pour the mixture through a fine sieve into a jug, to remove any stringy bits of egg.
Add the allspice, nutmeg, vanilla and salt to the custard and mix well.
Pour the custard evenly over the bread, then leave to stand at room temperature for 30 minutes before baking.
Bake the pudding at 170°C for 45-55 minutes, until golden and puffed. Allow the baked pudding to stand for 5-10 minutes before serving warm.