Wash the lemons hot, grate them dry, tear off the peel into thin strips with a zest ripper. Halve the fruit and squeeze the juice. Shake out elderflower umbels and wash briefly in standing water.
Let the flowers drip off on kitchen paper.
Bring sugar and 1 litre of water to the boil. Add flowers, lemon juice and peel. Leave the syrup to cool, then cover and leave in a cool, dark place for 3-4 days. Strain the syrup through a cloth and fill into bottles.
Close bottles well and put them in a cool place. The syrup keeps for about 6 months in the refrigerator. Fill the syrup with water (approx. 1 part syrup and approx. 4 parts water for 1 250 ml).