Slice two strips from the piece of cucumber and set aside. Grate the remaining cucumber into a sieve set over a bowl and press down on it with the back of a spoon to extract as much juice as possible. Add the elderflower bubbly and gin to the cucumber juice. Wind the strips of cucumber around the inside of your choice of glass, pour in a quarter of the cucumber gin mixture, and top up with a quarter of the bottle of tonic water to serve.