Combine the base ingredients and press onto base of 20cm/8in springform tin. Chill for half an hour.
Mix the gelatine with the water, stir well and let cool.
To make the filling: Put the cream cheese, sugar, and salt in the large bowl of an electric stand mixer fitted with a paddle or beaters, or in a large mixing bowl. Using the stand mixer or a handheld mixer on medium speed, beat the ingredients until smooth, stopping often to scrape down the sides of the bowl and under the blades with a rubber spatula. Turn the speed to high and continue to beat until the mixture is creamy. Stop the mixer and add the sour cream, gelatine, and vanilla; beat three minutes longer, again stopping as necessary to scrape down the sides of the bowl and under the blades.
Put the pieces of white chocolate in a small microwave-proof cup or bowl. Put the container in the microwave oven, cover with a paper towel, and heat for 30 seconds. Stop and stir the chocolate. Repeat the process once or twice more, just until the chocolate is completely melted and smooth.
Stir the melted chocolate into the cream cheese mixture and continue to beat until well combined and smooth. You should have about six cups of the mixture.
Pour half the cream cheese mixture into the springform pan. Sprinkle half the chunky chocolate biscuits over the mixture. Pour in the remaining cream cheese mixture, smooth its top with a rubber spatula, and then scatter the remaining chocolate biscuits evenly over its surface.
Place the cake into the fridge and leave to set for at least two hours, preferably overnight.
When ready to serve, remove the cheesecake from the refrigerator. Dip a long, sharp knife in warm water and run the knife around the inside of the springform pan to loosen the cake. Remove the outer ring. Continue to dip the knife into warm water as necessary as you cut neat wedges. To serve, place a wedge of cake on a cake plate and garnish, if you like, with fresh berries.