Preheat the oven to 160°C, gas mark 3. Line a 21cm loose-bottomed cake tin with baking parchment. Blitz the hazelnuts in a food processor, then add the biscuits and pulse until you have coarse crumbs. Tip into a bowl and mix with the melted butter. Spoon into the base of the cake tin and press down well.
Place the mascarpone cheese and soft cheese in a large bowl with 150g of the caster sugar, and mix together well. Stir in the flour, vanilla, the finely grated zest from the lemon and the lemon curd. Then beat in the eggs, one at a time, making sure one is incorporated before adding the next.
Pour on top of the biscuit base. Tap gently to level out the surface, then place in the oven and bake for 1 hour. Turn off the oven, open the door and leave the cheesecake inside to cool. When totally cooled place in the fridge and chill for at least 3 hours or for up to 24 hours.
Meanwhile, make the lemon drizzle syrup. Slice the lemon thinly and place in a small saucepan with the remaining caster sugar and 100ml of cold water. Bring to a gentle simmer for about 5 minutes until thickened and syrupy. Set aside to cool.
When ready to serve whip the cream with a hand-held electric mixer until just thick with soft peaks. Spoon over the top of the chilled cheesecake. Decorate with the lemon slices and drizzle over the lemon syrup.