Preheat the oven to 160°C (140°C Fan/Gas 3) and line a 20cm springform tin with baking parchment.
Mix the crushed digestive biscuits with melted butter and optional demerara sugar. Press firmly into the base of the tin and chill for 15 minutes.
Beat the cream cheese until smooth, then mix in the caster sugar, vanilla extract, lemon zest, and lemon juice.
Add the sour cream, cornflour, and eggs, mixing gently until fully combined without overmixing.
Pour the filling onto the chilled biscuit base and smooth the surface.
Bake for 50–60 minutes, until the edges are set but the centre still has a slight wobble.
Turn off the oven and leave the cheesecake inside with the door slightly open for 1 hour.
Refrigerate for at least 4 hours, preferably overnight.
Decorate with fresh berries, berry compote, or a light dusting of icing sugar before serving.