The day before you want to bake the cake, combine the raisins, sultanas, currants, mixed candied peel, glacé cherries, orange juice and 100ml brandy in a large bowl. Cover and leave to soak overnight.
Butter a deep 20cm round cake tin, and line the base and side with baking parchment. Beat the margarine, sugar, treacle and orange zest together in a large bowl using an electric whisk until smooth and fluffy. Gradually whisk in the milk and vinegar (the mixture will split at this point, but will come back together when the dry ingredients are added, so don’t worry). Mix the flour, xanthan gum, bicarbonate of soda, mixed spice and cinnamon together in a second large bowl. Gradually mix the wet ingredients into the dry until smooth. Heat the oven to 150C/130C fan/gas 2.
Fold the soaked fruit into the cake mixture until combined, then spoon into the tin and level the top with the back of a spoon. Bake for 45 mins, then reduce the heat to 140C/120C fan/gas 1 and bake for a further 2-3 hrs until the cake is deeply golden and firm to the touch (cover the top of the cake with foil if it starts to darken too much). The cake is ready when a skewer inserted into the middle comes out clean.
Poke holes all over the top of the warm cake using a skewer, then spoon over 4 tbsp brandy and leave to cool in the tin for 1 hr. Transfer to a wire rack, remove the parchment and leave to cool completely. When the cake is cold, wrap in a layer of baking parchment, then a layer of foil before storing. Will keep in a dry, cool cupboard for up to three months, or in the freezer for up to six months.