Strain the fruit through a sieve or jelly bag and weigh it. Tip the purée into a clean pan and add an equal amount of sugar, plus the gin, if using. Bring to the boil and simmer until the mixture is very thick. Line ramekins with baking paper or clingfilm, pour in the sloe mixture and leave to cool and set for 2 hours, before turning out of the ramekins. Store, wrapped, in the fridge.