Combine the sugar, maple syrup and butter in a pan and briefly bring to the boil, stirring occasionally. Remove the pan from the heat.
Stir in the oat flakes, mixed nuts and orange zest, transfer the mixture to the prepared tin, press down.
Bake for approx. 20 mins. in the centre of an oven preheated to 160°C. Remove from the oven, allow to cool on a cooling rack. Tip the tray bake out of the tin and onto a chopping board, cut into bars of equal size.