Preheat the oven to 170°C/fan 150°C/gas 3. Grease and line the base and sides of a 20cm deep-sided round cake tin with 5g butter and baking parchment. Place the dried fruit in a mixing bowl. Pour over boiling water to cover and leave to soak for 10 minutes. Strain the dried fruit through a sieve, pat dry on kitchen paper and leave to cool slightly.
Cream the remaining butter and sugar together in the bowl of a free-standing mixer until light and fluffy. While the mixer is still on, gradually add the eggs, one at a time, adding 1 tbsp flour with each addition until well mixed. Turn off the machine and, using a large metal spoon, fold the remaining flour and mixed spice into the cake batter. Add the soaked fruits and finely chopped ginger and mix to thoroughly combine.
Roll a third of the marzipan into a circle the same size as the tin. Spoon half the cake mixture into the prepared loaf tin, spread level and lay the marzipan disc on top. Cover with the rest of the cake mixture and spread level with the back of the spoon. Cook in the oven for 1 hour, then turn the oven temperature down to 150°C/fan 130°C/gas mark 2 and bake for a further 50 minutes, until the cake is golden and a skewer inserted into the middle of the cake comes out clean. Cover the cake loosely with foil if it appears to be browning too quickly. Remove from the oven and leave the cake to cool slightly in the tin, then turn out on to a wire rack to cool completely.
Heat the apricot jam in a small pan, then pass through a sieve to remove any lumps. Turn the cake so the top is facing up and brush with the jam.
Divide the remaining two-thirds of the marzipan into two equal parts. Roll one half out into a circle the same size as the top of the cake, then roll the other half into 11 round balls. Place the marzipan circle on top of the cake and, with a knife, gently score diamonds into the surface, then top with the marzipan balls. Preheat the grill to medium-high heat and place the cake underneath for 1-2 minutes to turn golden.