You could not fail but notice that the elderflower tress are now in full bloom and are right for the harvesting to make some cordial, so here is a fool proof method.
20 heads of elderflower
1.8 kg granulated sugar, or caster sugar
1.2 litres water
2 unwaxed lemons
75 g citric acid
1. Shake the elderflowers to expel any lingering insects, and then place in a large bowl.
2. Put the sugar into a pan with the water and bring up to the boil, stirring until the sugar has completely dissolved.
3. While the sugar syrup is heating, pare the zest of the lemons off in wide strips and toss into the bowl with the elderflowers. Slice the lemons, discard the ends, and add the slices to the bowl. Pour over the boiling syrup, and then stir in the citric acid. Cover with a cloth and then leave at room temperature for 24 hours.
4. Next day, strain the cordial through a sieve lined with muslin (or a new j-cloth rinsed out in boiling water), and pour into thoroughly cleaned glass or plastic bottles. Screw on the lids and pop into the cupboard ready to use.