Place the flour, caster sugar baking powder, pumpkin spice and salt in a large bowl. Mix.
Add the melted butter, then beat the eggs in a jug, add the milk and pumpkin purée and mix until smooth in the jug, before pouring into the bowl.
Mix until combined well.
Prepare your slow cooker by oiling the pot with a little vegetable oil, to prevent sticking.
Spoon the cake mixture into the slow cooker pot.
Boil the kettle, then measure the water out in a jug. Mix in the caster sugar and pumpkin spice and stir until the sugar has dissolved. Pour the mixture over the cake mixture. It will look odd but the sugar and water mixture will go under each the sponge as it cooks and become the sauce.
Cook on high for about two hours , until the cake has baked and risen and a cake tester or skewer comes out clean.
Serve warm, straight from the pot, with cream. Under the sponge you will have a rich glossy pumpkin spice syrup which you can spoon over the pudding cake.