Shake out the elder flowers well, remove as much of the green as possible and pour the juice. Add the lemon thinly sliced and leave to infuse overnight.
Drain through a filter cloth and squeeze well.
Blend the strawberries. Add with the citric acid and the jelling sugar and bring to a boil. Then boil for 4 minutes (to gel), skimming if necessary.
Fill the jelly into glasses and let them cool upside down.