Wash lemons and oranges hot, grate dry, peel into thin strips with a zest ripper. Halve the fruits 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, lemons and- orange 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 it into bottles. Close bottles well and put them in a cool place. To mix, fill the syrup with water in a ratio of 1 : 4 (1 part syrup and 4 parts water). The syrup will keep for about 6 months in the refrigerator.