Combine the fine biscuit crumbs with the melted butter until you have the consistency of wet sand. Line an 8 inch straight-sided springform tart tin with nonstick baking paper. Use the biscuit mixture to line the base of the tin, then transfer to the fridge to set.
Meanwhile, soften the gelatine in the lemon juice and a little water if necessary, for 5 minutes. Heat the mixture in a small saucepan over a low heat until the gelatine has dissolved. Place the chopped white chocolate in a heatproof bowl and sit over a saucepan of gently simmering water, stirring occasionally until smooth. Remove to one side to cool a little.
Beat together the low-fat cream cheese and the sugar in a large mixing bowl using an electric hand-held whisk for 2 minutes. Add the low-fat Greek yogurt and gelatin mixture, and mix again until smooth. Then fold in the cooled, melted white chocolate. Finally, add the vanilla extract and beat well.
Pour onto the biscuit base and tap gently on the side to release any trapped air bubbles. Transfer to the fridge and chill for 2-3 hours until set. When ready to serve, remove from the tin and remove the nonstick baking paper and cheesecake as one. Garnish the centre of the top of the cheesecake with the grated chocolate and sit the sugar rose on top if using. Cut into portions using a hot, sharp knife.