Place sultanas in a bowl and pour over boiling water to cover. Set aside for about 10-15 minutes to plump up, before draining well.
Roughly grind nuts in a food processor. Transfer to a large mixing bowl and mix in cinnamon, drained sultanas and honey.
Unwrap the filo pastry gently (make sure it has fully thawed) on a clean, flat, dry work surface. Place a 21-23cm square tin or dish on the filo and, using a sharp knife, cut filo pastry to fit the size of your tin/dish.
Preheat oven to 170degC/330 Fahrenheit. Brush melted butter on the base and sides of the tin/dish.
Lay one sheet of filo in the tin and brush with melted butter. Repeat nine more times, so you end up with ten sheets of filo (with butter between every sheet). Spread the nut mixture over the filo evenly. Layer remaining ten sheets of filo on top, brushing with butter in between each sheet. Brush the top with melted butter.
Use a sharp knife to cut the top layers of the pastry into 25 squares (this makes the baklava easier to cut once it is cooked).
Cook for 50 minutes until the pastry is golden. Meanwhile, make the syrup – combine all ingredients in a pot and boil gently for 15-20 minutes until it has reduced and is syrupy. Remove cooked baklava from oven and pour hot syrup over the top evenly, removing cinnamon stick and cardamom pods. Scatter with pistachios to garnish. Allow to cool for about 15 minutes before cutting into pieces and serving with Greek yoghurt.