Dissolve the sugar in the boiling water in a large bowl or pan; finely grate the rind of the lemons and add , along with the citric acid, to the syrup; thickly slice the rest of the lemons and throw them in; finally add the flower heads (snip as much of the stalks off behind the flowers as possible) ; cover and leave to steap for 48 hours.
After 48hrs fish out the solids and strain through some muslin or laundered cloth into sterilized bottles. Apparently it keeps for a good month if stored cool or can be frozen in plastic bottles.
I was pleasantly surprised by how nice it tastes after initially sniffing the flowers with low expectations. I have read it goes well with Sloe Gin so I think I'll give it a try.