Combine all the ingredients in a saucepan and bring to the boil.
Allow the cordial to cook for 10-15 minutes until the mixture is amber in colour and has thickened to a syrupy consistency.
Remove from the heat then allow to cool completely.
When the mixture is cool, strain the lemon zest out and pour into bottles.
(This cordial will last for 2-3 months if kept in a sterilised bottle in the refrigerator.)