Grease a 28 x 18 cm (11 x 7 inch) deep Swiss roll tin with butter and line the base with nonstick baking paper.
Place the butter, honey and maple syrup in a medium saucepan and heat, stirring continually, until the butter has melted. Add the cinnamon, dried fruit, seeds and nuts, stir the mixture and heat for 1 minute. Remove from the heat and add the porridge oats, stirring until they are well coated in the syrup.
Transfer the mixture to the prepared tin and smooth down with the back of a spoon to compact into the tin and level. Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 15 minutes until the top is just beginning to brown. Remove from the oven and allow to cool in the tin, then chill in the refrigerator for 30–60 minutes.
Turn out the chilled granola, upside down, on a chopping board, then carefully flip it back over to its correct side. Using a long, sharp knife (preferably longer than the granola itself), cut into 12 squares.