Prepare the coulis by placing the raspberries, blueberries and icing sugar in a mixing bowl and mashing with a potato masher until smooth. Press through a fine sieve into a medium saucepan. Bring the purée to a simmer over a medium heat. Cook for 2-3 minutes until slightly reduced. Remove from the heat and allow to cool to one side.
Pre-heat the oven to 170° C. Grease and line a 2lb loaf tin with nonstick baking paper.
Combine the butter, sour cream, sugar, flour, eggs and vanilla extract in a large mixing bowl. Beat until smooth using an electric hand-held whisk; 1-2 minutes. Spoon into the prepared loaf tin and bake for 25-30 minutes until a cake tester comes out clean from the centre of the cake.
Remove and allow to cool on a wire rack for 10 minutes before turning out from the tin and cooling completely. Adjust the sweetness of the coulis with the lemon juice if necessary. Once the cake is cold, slice into portions and serve on plates.
Drizzle the coulis over and serve with generous dollops of the Greek yogurt on top of the cake.