Bake the cake the day before you want to decorate it. Preheat the oven to 170°C/150°fan/325°F/gas mark 3. Grease a deep 20cm/8inch round cake tin and line it with a DOUBLE layer of baking parchment - this helps to stop the cake from catching round the edges.
Whisk together the flour, baking powder, bicarbonate of soda and mixed spice in a large bowl.
Add the cubed vegan butter and rub it in using your fingertips until the mixture resembles fine breadcrumbs and no lumps of butter remain.
Stir through the sugar, orange and lemon zest, mixed dried fruit, dried cranberries and glace cherries until well combined then stir in the milk to form a batter.
Spoon half of the batter into the tin and spread it level. Roll 200g of the marzipan out into an approx 18cm/7inch circle and place it on top of the batter. Spoon the remaining half of the batter on top of the marzipan and spread it level.
Bake the cake for 1 ½ - 2 hours until firm and springy to the touch and a skewer inserted into the centre but not into the marzipan comes out clean. Cover the cake loosely with tin foil after an hour if the top starts to get too dark.
While the cake is baking make the syrup. Place the rum/orange juice and sugar in a small saucepan and stir over a low heat until the sugar has dissolved. Set aside.
Once the cake is baked, remove it from the oven and brush the top generously with some of the syrup. Leave to cool in the tin for 10 minutes then very carefully turn the cake out onto a wire rack (it is delicate when warm so be gentle!).
Brush the cake all over with the rest of the syrup, you can turn it over and brush some on the underside if you like. Leave to cool completely then store in an airtight container.
The following day, use some of the remaining marzipan to make 11 equal sized balls. I used a scale to ensure they were all the same size - 15g per ball. Set aside.
Roll the rest of the marzipan out on a surface lightly dusted with icing sugar. Cut around the base of a 23cm/9inch round cake tin (or similar sized plate) with a sharp knife to get an even circle that is slightly larger than the width of the cake.
Place the cake top side down on a plate so the bottom is now the top. Use small scraps of marzipan to fill in any holes to create a totally flat surface. Brush the top of the cake with apricot jam then place the round of marzipan on top.
Use your fingers to crimp around the edge then place the marzipan balls on top in a circle. Use a chefs blowtorch to carefully toast the marzipan. (This step is optional!). Store in an airtight container at room temperature.