Sift the flour, baking powder, sugar and ginger into a large bowl. Add the breadcrumbs, sultanas, currants, suet and lemon zest. Mix with a wooden spoon.
Combine the egg and milk, add to the dry ingredients and mix well. Add a little more milk if necessary, then set aside for 5 minutes.
Lay a sheet of baking paper on a work surface, and form the mixture into a roll shape about 20cm/8in long. Roll up the pudding in the paper and fold up the ends – but do not wrap too tightly. Wrap in a tea towel, put it in the top of a steamer, cover and steam for 1 1⁄2 hours. Do not let the pudding boil dry – replenish with boiling water if necessary as the pudding cooks.
Serve with hot custard or fresh cream.