Chop the cucumber into a few chunks, then put in a blender or food processor with the wine and elderflower cordial. Whizz until well blended, then pour through a sieve set over a large bowl or jug. Tip the contents of the sieve back into the machine, whizz again until pulpy, then sieve again into the liquid already collected. Add 1-2 tbsp of the cucumber pulp from the sieve into the liquid and discard the rest. Chill until ready to serve.
To serve, fill glasses with a few ice cubes and some mint leaves. Pour in enough cucumber base to come halfway up the glass, then top up with soda water.