Add 150g whole-wheat biscuits, 1 tablespoon sugar and 6 tablespoons melted unsalted butter into a food processor and combine until moist. Pour into a 9-inch spring form pan and press the crumbs into the bottom of the pan and 1 inch up the sides with your hands. Bake at 180 degrees for 8 minutes. Once baked, allow it to cool at room temperature.
In an electric mixer fitted with a paddle attachment mix 1.1kg room temperature cream cheese with 300g of sugar, creamed on medium high speed until light and fluffy (about 5 minutes). With the mixer on low add 4 tablespoons/60ml of sour cream. Add 1 tablespoon lemon zest and 1 ½ teaspoons of pure vanilla extract. Pour mixture into the cooled biscuit base.
Bake for 15 minutes at 230°C, then lower the temperature to 110°C and bake for another hour and 15 minutes. Then turn the oven off, open the oven door and allow the cake to sit in the oven for a further 30 minutes. Take the cake out of the oven and allow it to sit at room temperature until completely cooled. Refrigerate overnight.
In a small pan add 340g redcurrant jelly and melt over a low heat. Pour over a mixture of 1 half pint sliced strawberries, 1 half pint fresh raspberries and 1 half pint fresh blueberries (about 600g in total). Add the warm jelly and toss until well mixed. Arrange the berries on top of the cake.