Place the white rum or vodka, whichever you are using, the strawberries or raspberries, whichever you are using, the lime juice and 100ml (3½fl oz) of the stock syrup in a blender and whizz until smooth.

Taste and add more stock syrup if necessary. Pour into tumblers half filled with crushed ice.

Stock Syrup

Place the caster sugar and the water into a saucepan and bring slowly to the boil, stirring to dissolve the sugar. When the sugar has dissolved, boil for two minutes and then allow to cool.