Sieve the flour, baking powder and bicarb into a large bowl. Add the breadcrumbs, ginger, allspice, cinnamon and ½ tsp fine salt and stir everything together. Wrap the end of the frozen butter in foil and, holding it by the foil end, coarsely grate the butter into the dry ingredients. Unwrap and discard the foil to grate the last piece. Fold everything together so the butter is evenly distributed. Mix in the stem ginger, eggs, treacle, sugar and milk. Spoon into the pudding basin, leaving a small gap for the sponge to rise, and cover with a disc of parchment. Seal the pudding basin with a tight-fitting lid or a double layer of foil, sealed tightly around the basin and tied with cook’s string. Set on top of the trivet, cover with a lid and steam for 3 hrs, topping up with water so it doesn’t boil dry.