Preheat the oven 180C/350F/gas mark 4. Grease and fully line a 20cm springform cake tin and set aside. Coat the blackberries in the 1 tbsp flour and set aside. This helps them to stay buoyant, although you do want some sinkage for this cake as it creates a lovely pudding-like base for the cake. Set aside. In a jug, mix together the milk and lemon juice/white wine vinegar and leave to stand. This makes the buttermilk, but if you have actual buttermilk to hand feel free to use that instead, using 125ml buttermilk to replace the 125ml milk.