Yes I wondered about banana too. I think some experimentation is called for! Okay here is the recipe:
Ingredients
For the pumpkin and apple puree:
About 500g pumpkin or squash (a large wedge of pumpkin or 1/2 a butternut squash)
4 dessert apples
For the pudding
100g soft unsalted butter
100g self-raising flour
100g wholemeal self-raising flour (Or 200g of either)
1tsp ground cinnamon
A good grating of fresh nutmeg
75g caster sugar
1 egg
1tsp vanilla extract
Method
For the pumpkin and apple puree, preheat the oven to 190C or Gas 5. Cut the pumpkin or squash into wedges, scoop out the seeds and place skin-side down in a roasting tray. Cover with foil and bake for 30 minutes. Meanwhile, peel, core and roughly slice the apples. Add to the roasting tray with the squash, re-cover and bake for a further 30 minutes or until soft. Scoop the pumpkin flesh from the skin. Puree in a blender with the apple until smooth. Leave to cool. You should have about 500g of puree; measure 200g for the pudding (they suggest freezing the rest for another pudding or using it for soup).
For the pudding, butter a 1 litre pudding basin. Combine the flours, cinnamon and nutmeg in a bowl. Beat the butter and sugar together in a mixing bowl until light and fluffy. Add the egg, vanilla and 1tbsp of the flour and beat well. Gently fold in the 200g of cooled pumpkin puree. Sift in the remaining flour and fold in gently until evenly combined.
Scoop the mixture into the basin and smooth the surface. Cover the basin with a piece of pleated foil and secure with string. Stand the basin in a large saucepan, pour in boiling water to come halfway up the sides and put the lid on. Bring the water to the boil over a medium heat, then turn the heat down to a gentle simmer and steam the pudding for 1.5 hours, topping up with boiling water as necessary.
Lift out the pudding basin and turn the hot pudding out onto a plate. Serve straight away with cream or custard (I like it with natural Greek yoghurt).
I hope that's not some terrible breach of copyright. Apologies to Nikki Duffy and River Cottage if so...