Put all fruit in a container, pour over the sherry and stir in the orange zest, cover with lid, and leave to soak for 3 days, stirring daily.
After 3 days, grease and line a 23cm (9 in) deep round cake tin with a double layer of greased greaseproof paper.
Preheat oven to 140 C / 120 C 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 oven for 4 to 4 1/2 hours or until the cake feels firm to the touch and is a rich golden brown colour. Check after 2 hours, and, if the cake is a perfect colour, cover with foil. A skewer inserted into the centre of the cake should come out clean when the cake is done. Remove from the oven and allow the cake to cool in the tin.
When cool, pierce the cake at intervals with a fine skewer and feed with a little sherry. Wrap the completely cool cake in a double layer of greaseproof paper and again in foil and store in a cool place for up to 3 months, feeding at intervals with more sherry.
Place the cake on a cake drum or board, or serving plate and spread the jam on top. Roll out the almond paste/marzipan to cover the cake entirely then transfer to the cake and smooth down gently over the top and sides; trim away any excess. Repeat the process with the royal icing, using a clean surface to roll it out, and place over the almond paste, smoothing down the top corners and sides. Trim any excess away and use offcuts to decorate if desired, dust with icing sugar. Slice and serve.