The cordial recipe that i use is:
30 heads elderflower
4 pts boiling water
1kg sugar
1pkt citric acid (50g)
3 lemons grated rind and roughly chopped
Dissolve the sugar and citric acid in the water. Add the lemons.
When tepid, add the elderflowers and steep for 2 days
Strain and bottle
Can be frozen
I also freeze elderflowers, but I vacuum pack them first, and the results are excellent!!