Preheat the oven to 180oC/160oC fan/350oF.
Grease and line a 23 x 33 cm (9 x 13-inch) rimmed baking tray with baking paper.
In a large bowl, mix the flour, cocoa powder, sugar, baking powder and bicarbonate of soda. Add the water, pumpkin puree and oil. Stir until smooth. Pour into the prepared baking tray and spread out in an even layer.
Bake for 20-25 minutes until a skewer inserted into the centre of the cake comes out clean.
Take a clean tea towel and dust with a generous layer of icing sugar. Tip the hot cake out onto it. Trim the chewy edges of the cake off to make rolling easier. Starting at the short side, roll the cake up (keeping the baking paper on it to stop it sticking to the towel) along with the towel, as though the towel is the 'filling' of the cake. Set in the fridge for at least 6 hours to cool completely.