Soak the sultanas overnight in the rum. Next day drain the sultanas. Set aside the sultanas and the rum, in separate bowls.
Make the butternut squash purée: cut the squash in two horizontally and remove the seeds and stringy parts. Bake in a preheated oven (170C/325F/Gas 3) until cooked and it is beginning to collapse. Depending on the size of the squash this will take about 1 hour.
Scoop the flesh out of the skin and if it is still very wet, put the pulp into a saucepan over a low heat to dry it out. This could take from 15 minutes to 45 minutes depending on the amount of water the pumpkin contains. Stir the pulp from time to time. Don't let it scorch.
Blend the pulp in a food processor until smooth, then return to the pan to allow any moisture to evaporate. Allow to cool, then refrigerate until ready for use.
Increase the oven to 180C/350F/Gas 4.
Combine 200g/7oz sugar with the rum (used to soak the sultanas) and the orange juice and gently heat to dissolve the sugar.
Butter and flour a 23cm/9in spring-form cake tin. Combine the drained soaked sultanas, nuts, grated peel, cherries, cranberries, dates and grated butternut squash with 1 tbsp flour and set aside.
Beat the remaining butter with an electric whisk until light and fluffy. Slowly beat in 400g/14oz sugar. Add the eggs one at a time, beating thoroughly after each addition.
Fold in the butternut squash purée and spices.
Sift the remaining flour with the baking powder and salt.
Combine the bicarbonate of soda with the buttermilk.
Stir one third of the flour and one third of the buttermilk, into the squash purée mixture and repeat in the same quantities until the ingredients are used up.
Finally add the dried fruit and nut mixture to the cake mix.
Spoon the mixture into the spring-form tin. Bake for 1½- 2 hours or until a toothpick when inserted into the cake comes out clean. (You may need to cover the top of the cake three quarters of the way through with foil).
Remove from the oven and allow to rest for 10-15 minutes. Remove foil.
Prick the cake all over and dribble the rum mixture over the cake while it is still warm. Allow to cool and dust with icing sugar and serve with whipped cream.