I'm brewing a sparkling elderflower drink that relies on the natural yeasts present on the flowers/stalks to carbonate the drink after it has been transferred to bottles (swing-top type). There is sufficient sugar in the recipe, and this has worked for me in the past, but I've read that this sometimes doesn't work. Is there a way to carbonate an already-bottled drink that is flat, without affecting alcohol content or flavour?

The ingredients for 4.5 litres is:

* 7 elderflower heads
* 750 g sugar
* 2 lemons
* 2 table spoons wine vinegar