Heat the oven to 180C/ 160C fan/ gas 4. Line the base of a 20cm springform tin by putting a square piece of parchment paper or foil on top of the tin base and then clipping the side on so the paper or foil is trapped and any excess sticks out of the bottom.
For the crust, melt 85g butter in a medium pan. Stir in the darker biscuit crumbs so the mixture is evenly damp. Press the mixture into the bottom of the pan. Melt the remaining butter and stir in the digestive crumbs as before. Make a layer on top of the first, tamp everything down firmly and bake for 10 minutes. Cool while preparing the filling.
Increase the oven temperature to fan 220C/ 200C fan/ gas 7. Whisk the soft cheese with the caster sugar, plain flour and a pinch of salt, and vanilla extract. Whisk in the eggs and soured cream but don't over-beat. The batter should be smooth and and very thick. Divide the batter equally between 6 bowls and colour each one using purple, blue, green, yellow, orange and red.
Brush the sides of the tin with melted butter and put on a baking sheet. Spoon in the filling, starting with red and then orange, yellow etc. The mixture should be thick enough to spoon and spread each layer carefully on top of the other. Bake for 10 minutes.
Reduce oven temperature to 110C/ 90C fan/ gas ¼ and bake for 35-40 minutes more. If you gently shake the tin, the filling should have a very slight wobble in the centre, if it looks liquid, cook for a little longer and check again. Turn off the oven and cool with the door very slightly ajar. Chill the cooled cheesecake until you want to serve it. Decorate just before serving.
To serve whip the cream until it is thick enough to pipe. Spoon in the cream and pipe blobs or rosettes on top of the cheesecake. For rainbow rosettes, using a paint brush make some rainbow stripes of food colour down the inside of a piping bag fitted with a nozzle before spooning in the cream.