Line a 20cm square baking tin with parchment paper.
Put the sour cherries and blueberries in a pan with the cordial and 30ml of water and bring to a simmer then remove from the heat and set aside to cool and soak for at least an hour.
Pour the condensed milk into a pan, and heat until just coming to a simmer, stirring. Put the dark and milk chocolate in a bowl with the butter and pour over the hot condensed milk.
Stir until you have a smooth glossy mix.
Strain the fruit, discarding any excess liquid, then add to the chocolate mix with the biscuits and mini marshmallows. Stir with a wooden spoon until completely combined.
Tip it into the lined baking tin and spread out with the back of a spoon. Melt the white chocolate, then drizzle it all over the surface. Chill in the fridge for at least 3 hours until solid. Cut into 16 squares and serve.