Put all of the ingredients, apart from the brandy and rum, if using, into a very large mixing bowl and mix together. Cover and leave overnight in a cool place (but not the refrigerator).
Preheat the oven to 120°C/ Fan 100°C/Gas ¼. Line a large deep roasting tray with baking paper.
Place all the mincemeat mixture into the prepared tray and bake for 1 hour, stirring every 20 minutes.
Remove from the oven and leave to cool for about 30 minutes.
Stir in the brandy and rum, if using, and pack into sterilised jars. Store in a cool, dark place for up to 3 months.