Preheat your oven to 180°C (350°F) and line your traybake pan with baking paper.
In a large mixing bowl, cream together the softened butter and brown sugar until light and fluffy.
Add the eggs one at a time, mixing well after each addition, then stir in the orange zest.
Sift in the self-raising flour and mixed spice, and fold in gently until just combined.
Stir in the orange juice, mixed dried fruits, candied peel, and nuts until evenly distributed.
Pour the batter into the prepared pan and smooth the top with a spatula.
Bake in the preheated oven for 25-30 minutes or until a skewer inserted in the center comes out clean.
Once baked, allow the cake to cool in the pan for about 10 minutes before transferring it to a wire rack.
Slice the cooled cake into squares and serve it on a festive tray.