My recipie is 30 flowerheads, 3 slized (scrubbed) lemons, 1.5 litres of sugar, 50g citric acid and 1.5 litres of boiling water. Basically boil the water, sugar and citric acid, put everything else in a steel pot or other container (perhaps rinsing the flowerheads first) and pour the boiling liquid over it. Put a lid in top, store cool temperature for a 3-4 of days. Filter through a cloth and freeze.
Incidentally, the same recipie can be used with meadowsweet (Filipendula ulmaria) (using a 50 flowerheads), and is in my mind even nicer. My SO recently found a recipie for meadowsweet that calls for cinnamon as well and is a bit diffent in proportions and procedure, but we haven't tried that one yet.
I also have a late 14th century recipie for cheese cake that calls for elderflowers...