Heat the oven to 160C/140C fan/gas 3. Butter and line a 900g loaf tin with baking parchment. Melt the sugar, butter and 225ml water in a large saucepan. Once melted, add the raisins, sultanas, currants and mixed peel. Bring to a gentle boil, and stirring occasionally, cook until reduced and softened, around 10-15 mins. Remove from the heat and leave to cool to room temperature.
Tip the flour, mixed spice, bicarbonate of soda and a pinch of salt into a large bowl. Add the cooled fruit mixture and stir, then add the eggs and mix to combine. Bake for 45-55 mins until dark brown and a skewer inserted into the middle comes out clean. Leave to cool in the tin for 10 mins, then transfer to a wire rack to cool completely. Cut into slices to serve.