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Put the dried fruit, zests and juice, rum, coconut oil and sugar into a large pan set over a medium heat. Give it a good mix, bring to the boil, then lower the heat and simmer for 5 mins until the sugar has dissolved. Tip the mixture into a large bowl and leave to cool for 30 mins.
Heat oven to 150C/130C fan/gas 2. Line a deep 20cm cake tin with a double layer of baking parchment, then wrap a double layer of newspaper around the outside, tying it with string to secure. Mix the chia seeds with 150ml water. Leave to sit for 5 mins until gel-like and thick.
Add the remaining ingredients to the fruit mixture, along with the chia seed mix, and stir well, making sure there are no pockets of flour. Tip into your prepared tin, level the top with a spoon and bake in the centre of the oven for 2 hrs.
Remove the cake from the oven, poke holes in it with a skewer and spoon over 2 tbsp of rum. Leave the cake to cool completely in the tin.
To store, peel off the baking parchment, then wrap well in cling film. Feed the cake with 1-2 tbsp alcohol every fortnight, until you ice it. Don’t feed the cake for the final week to give the surface a chance to dry before icing.