Grease a large baking tray and line with baking paper. Preheat the oven to 170°C, gas mark 3. Tip the granola onto the tray and spread out using a spatula. Cook for 40 minutes, stirring and breaking up after 20 minutes to ensure even browning. Reduce the oven temperature to 130°C, gas mark 1/2 and cook for 5-10 minutes more, until golden brown and crisp. Remove from the oven and leave to cool completely.