Pick your elderflowers on a sunny day just before noon for the best scent. Discard any with brown flowers.
Give each flower head a good shake to get rid of any creepy crawlies, although you will strain them out later.
Snip the tiny flower heads off the thick stalks, leaving only the thin stalks attached to the flowers.
Place in the sterilised jar, top with the lemon zest and pour the vodka in until it completely covers the lemon zest right to the limit of the jar. (This should prevent the flowers from turning brown but don't worry as an odd few will only result in a darker liqueur and it won't ruin the taste)
Place in a cool, dark place for two to four weeks.
Make a sugar syrup by gently heating the sugar with the water until the sugar has dissolved. Allow to cool before using.
Strain the elderflower, lemon and vodka liquid through a muslin cloth into a bowl.
Add half the sugar syrup and taste for sweetness before adding the remaining syrup if required. I use all of the syrup and I find it isn't overly sweet.
Decant into clean sterilised bottles and leave for for two months in a cool dark place to mature. (If you really can't wait, it is okay for drinking now but it's worth waiting the two months)
Keeps for years but I'm sure it won't be on your pantry shelf that long!