Place the flower heads in a large bowl together with the orange and lemon zest.
Bring 1.5 litres water to the boil and pour over the elderflowers and citrus zest. Cover and leave overnight to infuse.
Strain the liquid through a muslin and pour into a saucepan. Add the sugar, the lemon and orange juice and the citric acid.
Heat gently to dissolve the sugar, then bring to a simmer and cook for a couple of minutes.
Use a funnel to pour the hot syrup into bottles. Leave to cool then store in the refrigerator.