Position an oven shelf in the middle of the oven. Preheat the oven to 180C (fan 160C).
Put the digestives into a sandwich bag (worth doubling up) and batter with a rolling pin until crumbs. Or blast them in a food processor.
Line the base of a 23cm springform cake tin with parchment paper. For the crust, melt the butter in a medium pan. Stir in the biscuit crumbs and sugar so the mixture is evenly moistened. Press the mixture into the bottom of the pan (easiest thing to smooth it down with it the base of a pint glass) and bake for 10 minutes. Cool on a wire rack while preparing the filling.
For the filling, increase the oven temperature to 240C (fan 200C). In a big bowl, beat the soft cheese at until creamy. Keep mixing, gradually add the sugar, then the flour and a pinch of salt, scraping down the sides of the bowl. Don’t over-beat or the mixture will get too much air in it and you’ll lose that rough texture that cheesecake should have.
Add the vanilla, lemon zest and juice. Grab a whisk and mix in the eggs and yolk, one at a time, scraping down the bowl and whisk. Stir the soured cream until smooth, then slowly mix in. Whisk to blend, but don’t over-beat. The batter should be smooth, light and somewhat airy.
Brush the sides of the springform tin with melted butter and put on a baking sheet. Pour in the filling – if there are any lumps, sink them using a knife – the top should be as smooth as possible. Dropping the pan flat from a couple of inches will help any air rise to the top.
Bake for 10 minutes. Reduce oven temperature to 110C (fan 90C) and bake for 25 minutes more. Turn off the oven and let cool in the oven (with the door shut) for 2 hours. The cheesecake may get a slight crack on top as it cools.
Cover loosely with foil and refrigerate for at least 8 hours or overnight.
Run a round-bladed knife around the sides of the tin to loosen any stuck edges. Unlock the side, slide the cheesecake off the bottom of the tin onto a plate, then slide the parchment paper out from underneath.