In a large bowl, combine the flour, Scottish oatmeal, suet or butter, sugar, ginger, cinnamon, mixed spice, currants, sultanas or raisins, baking powder, and baking soda. Stir everything together so the fruit and spices are spread evenly through the mixture.

In a separate bowl, mix the black treacle, eggs, and milk. Then pour the wet mixture into the dry ingredients and stir only until the batter comes together. Set aside.

Place a clean cotton tea towel in a colander in the sink and pour boiling water over it. Wring out the excess water and dust the cloth with a light layer of flour, shaking off any extra.

Set the floured cloth in a bowl and spoon the mixture into the center. Gather the cloth and tie it tightly, leaving room for the pudding to swell.

Place a small heatproof plate in the bottom of a large pot, lower in the dumpling, and pour in boiling water until it is just covered. Bring to a gentle simmer, keeping the lid slightly ajar, and simmer for 3 hours, topping up with boiling water as needed.

Lift the dumpling into a colander to drain, keeping the cloth on. Preheat the oven to 350°F.

Cut the string and slowly peel back the cloth, supporting the skin. Let it rest for 5 to 10 minutes, then transfer to a baking tray and bake for 5 to 10 minutes until the skin darkens.

Slice and serve the clootie dumpling warm with custard or cream.