Preheat oven to 180 degrees C. Line a 20x30cm pan with baking paper and lightly grease. Sift together flour, bicarb soda, ginger, cinnamon, nutmeg and allspice.
Beat together egg and water in a large bowl then whisk in white sugar and molasses. Fold in the sifted dry ingredients until smooth then pour batter into prepared pan.
Bake in preheated oven until the cake springs back to the touch, about 15 minutes. When done, remove from oven and allow to cool for 15 minutes. Then cover pan with a moistened towel and refrigerate until cold, about 2 hours.
To assemble remove cake from the pan,and invert onto a fresh sheet of baking paper; remove and discard the old baking paper. Sprinkle evenly with icing sugar then spread evenly with whipped cream. Roll up from narrow end to narrow end, forming a firm spiral log. Wrap in the baking paper and chill until ready to serve.