Put all the dried fruit in a container, pour over the sherry and stir in the orange zest. Cover with a lid, and leave to soak for 3 days, stirring daily.
Grease and line a 23cm (9in) deep round tin with a double layer of greased greaseproof paper. Preheat the oven to 140C, 120C fan, gas 1. Measure the butter, sugar, eggs, treacle and almonds into a very large bowl and beat well. Add the flours and mixed spice and mix thoroughly until blended. Stir in the soaked fruit. Spoon into the prepared cake tin and level the surface.
Bake in the centre of the preheated oven for 4-4½ hours or until the cake feels firm to the touch and is a rich golden brown. Check after 2 hours, and, if the cake is a perfect colour, cover with foil. A toothpick inserted into the centre of the cake should come out clean. Leave the cake to cool in the tin.
When cool, pierce the cake at intervals with a fine skewer and feed with a little extra sherry. Wrap the completely cold cake in a double layer of greaseproof paper and again in foil and store in a cool place.
Additional notes: To prepare the Christmas cake ahead: Prepare the fruit and soak in sherry 3 days ahead - this is essential to plump up and flavour the fruit. Make the cake and wrap as in stage 4. Store in a cool place for up to 3 months, following stage 4. You could also freeze the cake before decorating, for up to 3 months; defrost at room temperature.
Decorate with almond paste and royal icing.
