Pick elderflower heads that are fully open and seem freshly opened (ie. not full of browning flowers)
Shake off any insects and place in a large bowl with the zest of your oranges and lemons. I prefer to peel off the zest with a potato peeler for this recipe so I can return it to the mix when adding sugar.
Pour over enough just boiled water to cover completely.
1.5-2 litres usually does it.
Cover and leave overnight (or minimum 4 hours).
Strain the liquid through muslin, a clean cloth or jelly bag gently squeezing to extract all the liquid. Reserve the zest where possible.
Measure your liquid. You will need 350g sugar for every 500ml of liquid.
Combine in a large pan with the juice of your oranges and lemons. Add the zest back into the pan.
Bring to a gentle simmer, stirring to dissolve the sugar. Skim off any scum that is produced.
Cool and strain through muslin again. Pour into clean bottles and seal. Alternatively, freeze in small plastic containers.
I like to make this into ice-cubes which can be added to glasses of water.
To serve: dilute to taste, about 1 part cordial : 5 parts water.
Or 5 ice cubes to a pitcher of water.
You can continue to boil this mix till you get a thick syrup which can be used on ice cream or is great to add to anything made with gooseberry!