Preheat the oven to 180ºC/350ºF/gas 4 and line a 20cm x 35cm baking tray with greaseproof paper.
Put the shortbread, sugar and 2 teaspoons of the ground ginger into a food processor and whiz to fine crumbs. Remove 100g of the mix and keep this to one side.
Add the remaining teaspoon of ginger to the processor, then roughly chop and add the mixed peel and ginger, followed by the flour and baking powder. Pulse until well mixed.
Melt the syrup, treacle and butter together in a large pan. Once melted, stir in the mixture from the food processor until thoroughly combined.
Tip onto the baking tray and spread out evenly. Press the mixture down into the tray, using your fingers or the back of a spoon. When the mix is a flat, dense and even layer, pop the tray in the preheated oven for 8 to 10 minutes.
Take the tray out of the oven and sprinkle the hot gingerbread with the reserved crumbs, pressing them down really well with a potato masher or spatula.
Carefully cut into good-sized pieces with a sharp knife, and leave to cool in the tray before eating.
