Preheat the oven to 190°C/375°F/Gas mark 5. Grease a 23cm/9in round cake tin.
Sift the flour in a large bowl, and rub in the butter until the mixture resembles fine breadcrumbs, then mix in the sugar and the ginger.
Dissolve the bicarbonate of soda in the warm water. Stir in the sour milk and molasses. Add to the flour mixture to make a fairly sticky dough.
Work the dough thoroughly, and put in the prepared cake tin. Bake in the oven for 50 minutes. Leave to cool in the tin for 30 minutes before turning out to cool completely