Soak broken gelatine leaves in enough water to cover for 90 minutes.
Simmer blackberries, water, sugar, and lemon juice in a pan until fruit breaks down.
Strain the mixture through a muslin-lined sieve into a bowl without pressing the pulp.
Warm the collected liquid in a clean pan and whisk in the soaked gelatine.
Remove the pan from heat and stir in the sloe gin.
Pour the mixture into a 700ml mould and chill in the fridge overnight.
Dip the mould into hot water for 20 seconds to loosen the jelly.
Invert the mould onto a plate and carefully lift to release the jelly.