Sorry I didn't give you this last night - there was a strange redirect when trying to connect. Anyway, here goes ...
It isn't perfect, as the ratios of sugar to acid to flowers won't allow that without MASSIVE dilution (in which case you may as well just have started from scratch). I've used Andy's champagne recipe as a target. The end product is a bit heavy on the flower side, but as it's also heavy on the acid side that should cancel out. If you can accept that, it all becomes pretty simple. You'll end up with a maximum of 5% ABV.
So, for each LITRE of cordial, add a tablespoon of mild white wine vinegar and then make up to 7 litres with tepid water. That's it for adjustments. Now we come to decision time - to yeast or not to yeast. You can then leave it in a covered bucket in the hope that a wild fermentation will begin (that's the traditional way) or you can introduce your own yeast. If you decide to add yeast, then you have another decision - how much. A wild fermentation would start with only a few yeast cells getting in. If you want to imitate that as much as possible, then add a pinch of yeast. If you want to be doubly sure, add a level teaspoon - but be aware that this will soon turn into a lot and WILL make your champagne cloudy.
I'll finish this with Andy's words (so he'll be the one to get sued

).
"Cover with a cloth and leave for between one and four days, until it looks like it's starting to bubble a bit. Strain through a muslin/cheesecloth into swing-top bottles and drink after about two weeks.
Caution: Elderflower champagne can be very volatile so shouldn't be stored in thin glass bottles. It should also not be stored in any place in which it can do damage. It is advisable to keep the bottles cool before opening."
So - VERY IMPORTANT - make sure that your bottles will take the pressure. Swing-top beer bottles, proper champagne bottles or PET bottles are suitable. NOTHING else is. Making elderflower champagne means that you are tightly capping an actively fermenting liquid - you have been warned.
Mike