Preheat the oven to 180C (160C in a fan oven) Gas 4.
To make the syrup, heat the sugar, water, lemon juice and orange blossom water in a small, heavy-based saucepan and cook over a medium heat for approximately 15 minutes, until the sugar has melted and a syrup has reduced by 1/3 rd, set aside to cool.
Grease a 17cm x 28cm (11in x 7in) baking tray with butter.
Place the nuts in a blender and blitz until coarse. Transfer to a bowl with the sugar and spice.
Lay 2 sheets of filo pastry, one at a time, into the tray, brushing each sheet with butter before adding the next. Pour half the nut mixture on top.
Layer up the next 2 sheets, brushing each sheet with butter, as before. Top with the remaining nut mixture and finally, top with the remaining 2 sheets of filo.
Using a sharp knife, cut a criss-cross (diamond) pattern into the top layers of the pastry.
Place in a preheated oven for approximately 20 minutes, decrease the temperature to 150C/Gas 2 and cook for a further 30 minutes, or until the pastry is slightly puffed and golden on top. Remove from the oven and allow to cool slightly. Pour half of the cooled syrup over the baklava and allow to stand for 5 minutes before pouring over the remaining syrup. Leave for a further 10-15 minutes before cutting into small diamond-shaped pieces.