Make this every summer, its lovely, I freeze in plastic bottles. Its great with a splash of Gin and sparkling water. The citric acid is becoming more difficult to purchase, however if you have a regular pharmacy they should sell it to you.
20 Elderflower Heads
4lb sugar
2 pints boiling water
2 3/4 oz of citric acid
2 lemons grated/sliced
Dissolve sugar in water
Add citric acid
Pour over elderflowers
Add Lemons
Leave covered overnight
Strain in Jelly bag
Bottle
Enjoy
p.s. the flower heads are also lovely dipped in a light batter and dep fried, dust with icing sugar