Remove redcurrants from stems, wash and put in a pan with 2 tbsp water. Bring to the boil, lower the heat, cover and simmer for 5 mins until softened. Push through a sieve to make a purée. Stir in the elderflower cordial and set to one side to cool.
Put the caster sugar in a pan with 300ml/½pt water and leave over a low heat for 5 mins until the sugar dissolves. Raise heat and boil for 10 mins.
Tip redcurrant mixture into syrup and mix. Return to the boil, turn down and simmer for 2 mins. Cool, pour into a container and freeze for 3-4 hrs until frozen. Scoop into glasses and top with redcurrants.