If you can see what you think are Elderflowers this early then they are not elderflowers... The common mistake is to confuse Elderflowers with 'cow parsley'. The flower is very similar but the 'cow parsley' is out in May so you may have seen these.
DO NOT ATTEMPT MAKING CHAMPAGNE WITH COW PARSLEY... It won't work and whatever resides in the bottle will taste disgusting. Cow parsley usually grows at side of the road and has a long green stem, it doesn't get very tall either, (1m apx). Elderflower grows on trees so if it's in any way mature it'll have brown bark.
I gave elderflower champagne a go last year and I just relied on wild yeasts present in the flower. Unfortunately my batch didnt ferment, yeats must have died...
However the drink was still very pleasant when supped cold, rather like a flat lemonade. I used the Fearnely Whittingstall recipe which I'd reccomend.