Combine fruits, nuts and rind in large bowl; mix well. Sprinkle castor sugar evenly into heavy-based frying pan, place over medium heat, cook until sugar begins to melt and brown, gently stir until completely melted and golden brown. Remove from heat, add orange juice, return to heat, stir constantly until toffee pieces are dissolved. Do not boil as this will evaporate too much of the liquid. Add Grand Marnier, strain liquid to remove any small pieces of toffee; cool to room temperature.
Place fruit mixture in airtight container or large jar with tight-fitting screw-top, pour Grand Marnier mixture over fruit mixture. Seal, stand overnight. Next day, invert jar or mix fruit mixture well. Do this for 10 days (up to a month is fine).
Beat butter until soft, add brown sugar, beat until combined. Add eggs one at a time, beat until combined before adding next egg. Pour fruit mixture into large bowl, add creamed mixture, mix well. Add sifted flour, mix well. Spread mixture into deep 19cm square or deep 23cm round prepared cake pan. Bake at 150°C for 3 to 3½ hours.
To decorate cake with marzipan oranges: Use a packet of almond paste. Dust hands with pure icing sugar. Roll pieces of paste into balls, place small part of clove on orange. Use skewer to roll orange over a grater to give effect of skin. Stand oranges on wire rack to dry overnight. Paint with orange food colouring.