To make the cordial, heat the oven to 180C/fan 160C/gas 4. Put the blackberries, apple chunks, sugar, lemon and mint in a dish with 400ml water and bake in the oven for 30-45 minutes, until the fruit gives up its juice and the sugar has dissolved. Strain and leave to cool.
To make the cocktail, put the gin and cordial in a glass, fill with ice, then top with cider and gently mix together. Garnish with a thin slice of apple, a sprig of mint and a blackberry.