Finely grate the orange zest and squeeze the juice. Pour the juice into a measuring jug and top up with cold water to 200ml. Pour into a large saucepan and add the zest, prunes, mixed dried fruit and spice. Stir well and place over a low heat. Bring to a gentle simmer. Simmer gently for 3-4 minutes until the liquid is almost all absorbed, stirring occasionally. Remove from the heat and leave to cool for 30 minutes. Preheat the oven to 160°C/fan 140°C/Gas 2 ½. Butter and line the base and sides of a 20cm round deep cake tin with two layers of nonstick baking paper, greasing with a little extra butter between each layer of paper to help it stick. Whisk the eggs together in a large bowl then stir in the butter, milk, flour and baking powder. Beat with a wooden spoon until smooth. Stir in the soaked fruit until thoroughly combined. Spoon into the prepared tin and level the surface with the back of a spoon. Cover the tin loosely with a piece of aluminum foil. Bake in the centre of the oven for 1 hour. Remove the foil and bake for a further 30 minutes or until the cake is pale golden brown and a skewer inserted into the centre comes out clean. Remove the cake from the oven and cool in the tin for 30 minutes. Carefully turn out, peel off the lining paper and leave to cool completely on a wire rack.