Preheat oven to 180 C / Gas 4. Line a 25x38cm swiss roll tin with baking parchment and lightly grease. Sieve together flour, bicarbonate of soda, ginger, cinnamon, nutmeg and allspice.
Beat together egg and water in a large bowl, then whisk in caster sugar and treacle. Fold in the sieved dry ingredients until smooth, then pour batter into prepared tin.
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 tin with a moistened towel and refrigerate until cold, about 2 hours.
To assemble, remove cake from the tin and invert onto a fresh sheet of baking parchment; remove and discard the old sheet. Sprinkle evenly with icing sugar, then spread evenly with whipped cream. Roll up from narrow end to narrow end, like a swiss roll. Wrap in the baking parchment and chill until ready to serve.