Remove 8 thin strips of zest from the lemon using a vegetable peeler and set aside. Thinly slice the lemon and place in a small pot with the sugar and 80ml water. Bring to a boil and cook until the sugar dissolves. Let cool and strain the syrup.
For each of 8 martini glasses, rub a strip of zest around the rim and then drop in the bottom of the glass for garnish. Combine the gin, elderflower liqueur, lemon syrup and ice in a measuring cup or pitcher and stir until very cold. Remove any unmelted ice or strain into the prepared glasses. Serve ice cold.