This year, I managed to coordinate the buying of the sugar and the finding of the citric acid, brought over from the UK by a friend (who later said she wondered what would have happened if security had come on all this white powder in her luggage).
Apart from that all you need are the elderflowers, oranges, lemons and a large pot. Chunk the fruit, add the flowers, sprinkle with citric acid and immediately pour on the sugar syrup you have simmering on the stove. Cover and leave in the fridge for four days, then strain and bottle. It's sharp, musky, and lovely with plain water, fizzy water or, as we found out last night, blanquette de Limoux. Next, gin.