Preheat the oven to 180°C/160°C fan/35°0F/gas mark 4. Grease an approx 11.5 x 21.5 cm / 4.5 x 8.5 in loaf tin and line it with a strip of baking parchment that comes a little way above the sides so that you can easily lift the cake out once it is baked.
To make the batter, whisk together the pumpkin puree, oil, milk, sugars, vinegar and vanilla in a large bowl.
Whisk in the pumpkin spice, baking powder, bicarbonate of soda and salt until well combined then add the flour and whisk gently until no dry lumps remain.
Transfer 450g (a bit less than half) of the batter to a separate bowl. Sift in the cocoa powder, add the milk and stir until combined.
Spoon a bit of the pumpkin batter into the lined loaf tin and spread it out. Top with some of the chocolate batter.
Repeat alternating the batters until you have used them all. Aim for 4-5 layers of each. Jiggle the tin to level the batter out.
Bake for about 75 minutes until a skewer inserted into the centre comes out clean. Loosely cover the top with tin foil for the last 20 minutes of baking if it starts to get too dark.
Leave the cake to cool in the tin for 10 minutes then carefully lift it out onto a wire rack and leave to cool completely before slicing.