Place 600g (1lb5oz) blackcurrants in a saucepan and add 150g (5oz) sugar, 200ml (7floz) water and 2tablespoon golden syrup. Heat gently stirring until the sugar dissolves then cook the blackcurrant for about 5 minutes until the fruit is very soft.
Strain the blackcurrants through a sieve pressing with the back of a spoon to remove as much of the fruit juice as possible. Allow to cool completely.
Stir in the cassis and pour into an ice cream maker and and churn until the mixture becomes a thick slush. Then transfer to a freezer container and freeze completely.
If you do not have and ice cream maker, pour the blackcurrant mixture into a shallow freezer container and place in the freezer freeze until it begins to freeze around the edges. Beat together with a fork to break up the ice crystals and return to the freezer. Repeat 3 or 4 times. Then allow to freeze completely.
To serve, remove the sorbet from the freezer about 10 minutes before serving.