Pre-heat the oven to 180°C/Fan 160°C/Gas 4. Line the base of a 23cm round springform cake tin: flip the base of the cake tin over and lay a piece of baking paper over it, then tuck this into the open springform ring and close it over the base (so the base is flat side up); trim off the excess paper.
Crush the digestive biscuits in a food processor (or you can pop them in a strong plastic bag and bash them gently with a rolling pin).
Mix the crushed biscuits with the melted butter, tip them onto the lined base of your tin and flatten down evenly with the back of a spoon. Place in the oven for 10 minutes to bake lightly, then remove and set aside to cool slightly, while you make the filling.
In a large bowl, beat the cream cheese to soften, using an electric whisk. Gradually beat in the sugar, whole eggs and extra egg yolk. Lastly, add the vanilla, lemon zest and juice, and the crème fraîche and beat until smoothly incorporated.
Pour the filling over the cheesecake base in the tin. Stand the cake tin on a baking tray and bake on a low shelf in the oven for about 45-50 minutes. Once cooked, the cheesecake should still have a slight wobble in the middle. Turn the oven off and leave the cheesecake inside to cool slowly, with the door slightly ajar.
Meanwhile, for the blueberry sauce, put the water and sugar into a small saucepan over a medium heat to dissolve the sugar. Bring to the boil and let boil for 2 minutes, then add the blueberries. Cook for 2-3 minutes, until the berries have released some colour into the syrup but are still plump. Stir in the cornflour mix and cook, stirring, until the mixture thickens. Pour the blueberry sauce into a shallow dish and leave to cool. Refrigerate until you need it.
When ready to serve, run a palette knife around the inside of the cake tin to loosen the sides of the cheesecake, then carefully release it from the tin. Transfer the cheesecake to a serving plate and spoon the blueberry sauce over the top. Cut into wedges to serve.