To make the ice cream, pour the cream and milk into a large saucepan. Bring to the boil, then reduce the heat and simmer gently for 2-3 minutes, stirring occasionally.
Whisk the yolks and sugar together until pale. Using a large ladle, gradually add the hot cream and milk mixture, whisking well between each addition. When thoroughly combined, return the liquid to the saucepan and set over a low heat.
Cook for 10-15 minutes, stirring constantly until the custard is just thick enough to coat the back of a spoon. Do not over-heat or the yolks will begin to scramble. Remove from the heat immediately and pour into a clean bowl. Stir in 100ml of the elderflower cordial. Cover with clingfilm. Leave to cool.
Taste the custard and add more elderflower cordial if the flavour isn’t pronounced enough, bearing in mind that it should be subtle, but that the flavour will be subdued by freezing. Pour the cooled custard into an ice-cream machine and churn until frozen. Spoon into a freezer-proof container and put in the freezer until needed