Preheat oven to 180 C / Gas 4. Grease a 20x30cm baking tin.
Cut filo pastry to size of tin, and place half the sheets in the bottom of the tin. Grind walnuts with 100g sugar and cinnamon in food processor or blender. Spread ground walnut mixture evenly over filo. Place remaining filo sheets on top of walnut mixture.
Melt butter with oil in small saucepan over low heat or in microwave. Brush melted mixture over top layer of filo. Cut baklava into diamond shapes with a sharp knife.
To make syrup: Combine 700g sugar, water and lemon juice in large saucepan over medium-high heat. Bring to the boil, then remove from heat. Let cool before pouring over hot baklava.
Bake the baklava for 30 minutes in centre of oven, or until golden brown. Pour cooled syrup over hot pastry. Let cool before cutting into small diamonds and serving.