I've just got another 50 elderflower heads and steeped them in boiling water, zest of 4 lemons and the juice of the lemons.
I was told to boil this mix tomorrow, simmer for 2 mins then add it to the too sweet mix to blend them both together.
Do you think this will work
Yes, that sounds like a fine idea. I could suggest two things to guide you to make success more likely.
(1) if you have access to a hydrometer (about $5 at a Home Brewing store), you could measure the specific gravity (SG) of the too-sweet mixture. You could also mix up a glass of something that you consider to have the correct sweetness, and measure its SG as well. Also, measure the SG of the new, unsweetened mixture, although this should be close to 1.000. Then you could use a
Pearson's square to figure out how much of each mixture to blend together to get the desired SG (which will correlate very closely with sweetness).
(2) if you don't want to do that, I would suggest making bench trials of mixtures of the two to figure out how much of each to use. Say, fill 1/3 of a glass with the unsweetened mix, then blend in more and more of the too-sweet mix until you like it, keeping track of how much you added.
Finally, a comment: This forum is called
winemakingtalk.com; in common parlance, a cordial contains alcohol; and some people who are grandchildren are of legal drinking age. Hence our confusion.