Put 200g of the (softer) blueberries into a pan with 200g of the sugar and 175ml of water. Heat slowly to dissolve the sugar, then simmer for 7–8 minutes. Mix the cornflour with 25ml of water and stir into the mixture. Simmer, stirring, for 2–3 minutes. Strain through a fine sieve into a bowl, pressing the berries in the sieve to extract as much juice
as possible. Leave to cool.
Line a 17–18cm springform cake tin with greaseproof paper. Crush the biscuits in a food processor to coarse crumbs. (Or put in a plastic bag and smash with a rolling pin.) Mix with the butter and pack into the cake tin to make a firm base using the back of a spoon.
Whip the cream and sugar together until fairly stiff. In another bowl, beat the cream cheese to soften, then fold in the cream with the lemon zest and vanilla extract. Lightly fold through half of the blueberry syrup to create a rippled effect. Spoon the mix onto the biscuit base and place in the fridge for 2–3 hours until firm.
Mix the rest of the fruit with the remaining blueberry syrup. To unmould the cheesecake, run a hot knife around the edge, then release the side of the tin and slide the cheesecake onto a board. Cut into slices and serve each one topped with a generous spoonful of blueberry sauce.