Grease the base of the cake tin, then line the base and sides with baking paper. Break the biscuits into chunks, then put in a food processor. Whizz until the biscuits have the consistency of sand, then tip into a bowl. Add the sugar, the melted butter and water, then mix until all the crumbs are damp with the butter, like wet sand. Tip the crumbs into the cake tin, then use the back of a metal spoon or a straight-sided glass to press the mixture into the base of the tin. Put in the fridge to set for an hour or so.
When you’re ready to bake, heat the oven to 170°C/150°C fan/gas mark 3. To make the filling, beat the cream cheese and sugar in a bowl, using an electric mixer, until soft and well combined. Add the soured cream and beat on a slow speed to combine. Don’t overbeat the mixture or beat it too fast, as this will beat in air and make the cheesecake rise as it cooks, then crack as it cools – always set the mixer speed to slow-medium. Add the eggs one at a time, beating in each one until incorporated. Add the lemon zest and juice, then beat slowly for about a minute.
Pour the mixture over the biscuit base. Bake for 1 hour or until the cheesecake is a light golden colour, but still a bit wobbly in the middle. Leave to cool in the oven with the door ajar. Refrigerate for 12 hours or overnight until chilled.
To make the lemon curd topping, put the butter and lemon zest in a heavy-based saucepan and put over a medium heat. In a separate bowl, mix together the egg yolk and sugar. When the butter has melted, add the egg yolk mixture and lemon juice, then whisk constantly using a balloon whisk until the curd thickens. This will take about 10 minutes. Leave to cool.
Just before serving, carefully remove the cheesecake from the tin, then spread with most of the lemon curd. Decorate with big dollops of thick cream and candied lemon slices, if desired. Drizzle with the remaininglemon curd, dust with icing sugar, then serve.