Mix the flour, sugar and fruit in a large bowl.
In a separate bowl, mix 2 cups of boiling water with the baking soda.
Add the flour and fruit mixture and stir or fold well.
Cover and leave overnight. Mix well the next morning.
Dampen a clean tea towel or pudding bag and sprinkle it lightly with flour.
Turn the pudding out of the bowl onto the towel and gather the towel together to form the pudding into a ball. Tie very securely with kitchen twine.
Place the wrapped pudding in a large saucepan and fill with water until just covered.
Bring to a boil, then reduce the heat and simmer for 3 hours, making sure the pudding is always well covered with water.
Cool slightly before carefully peeling off the towel so as not to disrupt the delicious, thick skin that forms.
Slice into wedges to serve.