It appears to be really easy requiring only a few elderflower heads, water, two lemons a bag of sugar and some wine vinegar - mix together, leave for a full day and bottle - job done.
What's the catch? Beware of explosions

Its all very well these rural folks spreading the delights of farmhouse recipes, nonchalantly laughing off the BOOM factor and resulting sticky mess, but what saving is there if the odd explosion wrecks a pair of new curtains and creates a sticky mess that needs cleaning.
It seems the over excitement and excessive effervescence of the drink is due to fermentation - the natural yeast on the elderflower reacts with the sugar to make carbon dioxide and alcohol.
Could this be a new method of making biofuel? I read about a landrover being driven on banana oil for thousands of miles see wiki landrover "history" (it must therefore be true) so it wouldn't be so extraordinary if we see elderflower gas coming to a filling station near you?