Preheat oven to 170 C / Gas 3. Wrap the outside bottom of a 25cm springform cake tin with foil to prevent water seeping in while baking.
Cream together ricotta cheese, cream cheese and sugar until smooth. Mix in the eggs one at a time. Add flour one tablespoon at a time, mixing well. Stir in the soured cream and vanilla. Pour mixture into prepared tin.
Place springform tin in a large roasting tin, place in the oven and pour boiling water from the kettle into the roasting tin halfway up the sides. Bake for 90 minutes.
After 90 minutes turn oven off, but leave the cake in the oven for another 60 minutes (do not open door).
Cool at room temperature for 60 minutes and then refrigerate. Serve when thoroughly chilled.