Pick over the elderflower heads, trimming the flowers away from the stalk and shaking out any insects.
Put the sugar in a large saucepan and add the measured water. Heat over a low heat, stirring occasionally, until the sugar has completely dissolved. Bring to the boil and add the gooseberries.
Simmer for 7–8 minutes, then take off the heat.
While the gooseberries are simmering, finely pare the rind of the lemon(s) and slice the flesh. Swirl the elderflowers gently in a bowl of cold water.
Remove the elderflowers from the water and add to the sugar syrup along with the lemon rind and slices and the citric acid, if using. Stir, lightly cover with a clean tea towel and leave in a cool place overnight.
Strain the cordial through a fine sieve into a wide-necked jug or bowl. Pour the cordial through a funnel into 2 x 75cl) sterilized glass bottles and seal. Store in the refrigerator and consume within a month. You can also freeze the cordial in plastic bottles, though remember to leave some headspace to allow the liquid to expand.
To serve, dilute to taste with soda or sparkling water.
Note: using citric acid will increase the shelf life. Tip from Fiona: "You can make the cordial without citric acid if you’re not intending to keep it for long, but don’t overdo the lemon, otherwise it will just taste like lemonade."