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.
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.