Mary Berry's elderflower cordial
Put the sugar and 1.5 litre/3lb 5oz water into a large pan. Bring to the boil, stirring, until the sugar has dissolved. Remove from the heat and cool.
Slice the lemons thinly. Put into a large plastic box or a bucket.
Add the elderflower heads to the lemons with the citric acid and the Campden tablets. (CAUTION: Campden tablets produce an irritant gas if inhaled. Keep your face well back from the mixture.) Pour over the cooled sugar syrup. Cover and leave overnight or up to a couple of days.
Sieve and strain through muslin into sterilised bottles and store in the fridge. To serve, dilute to taste with still or sparkling water.
Look for elderflower heads from about the end of May. They can be frozen if liked: freeze about 25 heads in a bag and add to the hot sugar syrup straight from the freezer - this will stop them turning brown once defrosted. Campden tables are used in home wine and beer making to kill bacteria and also to inhibit wild yeasts. Citric acid is a natural preservative and also gives a slightly sour flavour to the cordial. Look for them in pharmacists.