Place 1kg of granulated sugar in a bucket or basin with 1 litre of boiling water and stir to dissolve. Add 50g of citric acid (or juice 2 large lemons) and add the zest of 2 large lemons. Gather 15 Elderflower heads and shake them to remove any insects and add the flowers to the sugar syrup. Cover and leave to stand for 1-2 days, stirring morning and night. Strain the Elderflower cordial through a muslin and decant into bottles - it is now ready to use! Elderflower cordial can also be purchased ready-made from supermarkets.